These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Pokémon Diamond and Pearl
Accidental Innuendo: So, in Platinum, you get a villa. You can buy furniture for it. When you mix records, sometimes your friends will find a TV program that is examining furniture you have. Sometimes they say that you have a "nice rack".
REGIGIGAS can't get it going because of its Slow Start!
The evolutions of previous Gen Pokémon (especially Gen 1 Pokémon like Rhydon) got heavy flak when first introduced, often called unnecessary and abominations of the older Mons. Over time, more people joined the side that recognized them as redemptions to Mons considered mediocre to forgettable beforehand.
Though their Breather Boss status varies slightly depending on which version you're playing. Aaron is slightly less so in Platinum, since he doesn't have Beautifly and Dustox anymore, though he's definitely still the easiest Elite 4 member. Candice and Volkner are slightly more so in Platinum, since their teams are actually all one element now.
Byron is definitely this if you picked Chimchar. Considering you can get a free Flamethrower TM as soon as you can use Surf, and you'll almost definitely have an Infernape with Close Combat by the time you battle him, you can beat him in three turns without him even getting to attack.
Pretty much all of the starters have an easy time against Byron, considering how they're very unlikely not to be in their third form by then. Torterra gets earthquake upon evolving, and while Empoleon may have a hard time with the Magneton Byron has in Platinum, Surf (which you need to get to Canalave in the first place) can still KO his other Pokémon easily.
Complacent Gaming Syndrome: The move Stealth Rock in the Meta Game. In competitive battling, it's almost required that you have both a Pokémon who can set up Stealth Rock and a Pokémon who can counter it when your opponent inevitably does a team whose typing takes minimal damage from it, as it is far easier to get up and keep up than to remove.
Crosses the Line Twice: A variation from the fandom regarding Drifblim, with Hilarious in Hindsight thrown into the mix. Nicknaming either it or its prevo "Hindenburg" (and having either one of them with the Explosion attack) was already a pretty common joke, but then they got an exclusive Dream World Ability, Flare Boost, that increases their Special Attack while Burned — and right in step with the games that debut the series's equivalent of America, too. Oh, the Humanity!...
Game Freak themselves seemed to want the players to feel sorry for him by adding his grandfather to Platinum and telling us about his sad, oppressive past. Of course, not too much, since we still want to beat him and put a stop to his plans...
Ensemble Darkhorse: Looker. Also Lucario, thanks to its appearance in the eighth movie and (later) Super Smash Bros. Brawl. The fact that it performs quite well in competitive battling also helps.
Riley counts because he's a pretty boy with blue hair and a trenchcoat. Just see how much fanart he gets compared to the other trainers you end up teaming up with.
Cynthia is easily the most popular champion in the series.
Garchomp, as mentioned under Base Breaker above, except now it was given a new secondary ability that makes it not so much of a Game Breaker.
Evil Is Cool: Cyrus may have been beyond redemption, but some find him to be badass.
Evil Is Sexy: Pretty much all the Team Galactic members (with the exception of Charon) are really hot. Cyrus is an unusual case because he isn't especially good-looking, yet he still manages to be very attractive in other ways.
Freud Was Right: Palkia is often mocked for having a long, streamlined neck and head with perfectly round shoulders...
Fridge Brilliance: Dawn and Lucas' Platinum art has them holding Repeat Balls, because they are repeating their adventure!
Game-Breaking Bug: Chatot is not exactly a Game Breaker on the battlefield. However, the cry changing properties of Chatter can actually break the game due to glitches, and therefore it is banned from use in Random Wifi.
That, and probably the fact that some people would use it to scream obscenities at their opponents.
Genius Bonus: Empoleon is the same height as Napoleon Bonaparte (Care to guess what Empoleon's name was based off of?).
In Japan, Empoleon is known as "Emperte", so the theme is the same, except just using Bonaparte's last name instead.
Internet Backdraft: The subject of Stealth Rock seems to be getting a bit of this. It doesn't help now that it's really easy to get in HeartGold and SoulSilver.
Just Here For The Legendaries: The Legendaries of this generation, particularly the Dragon Trio and Darkrai, are well-loved by the fanbase. However, most of the new non-Legendary Pokemon suffered from over-complicated designs and poor stats, except for a few such as Lucario, Garchomp and the previous-gen evolutions.
Base Breaker: Some fans prefer the designs from Generation IV onwards and/or think they are as complicated as the designs from previous generations.
Magnificent Bastard: Cyrus; he's a charismatic leader, a master of exploiting emotions, and always one step ahead of you.
Moreso in Platinum than in Diamond or Pearl, where he makes the stupid mistake of letting you free the Lake Trio, who then proceed to destroy his plans as a result. In Platinum, he's prepared for this, making two Red Chains instead of one so that he can bind both Dialga and Palkia, whose combined power blocks out the Lake Trio when they try to save the day.
Moral Event Horizon: Cyrus' entire plan and the measures he takes to accomplish it, all Jerkass Woobie traits aside. Charon's plan for Heatran probably would've also qualified had it actually gotten off the ground.
Nightmare Fuel: In Platinum when Giratina screeches and launches itself at the screen. Especially scary if you read a lot of Pokemon creepypasta, in which loud screeches and the screen being covered in darkness is a common theme.
Cyrus, particularly his scheme to annihilate reality and reshape it as he sees fit - "a world without spirit", as he himself puts it.
Padding: The initial pair (fixed in Platinum and HG/SS fixing it almost entirely) makes everything take forever to play out, even with animations turned off.
This was likely because Game Freak had obviously had problems programming on the DS. For instance, the glitches in the 3D overworld and the drastically lower framerate despite being on far more powerful hardware than the previous titles!
Rescued from the Scrappy Heap: Its evolution Bibarel can learn all TMs except Fly. Makes him the best HM slave in the whole game and one of the best in the series due to its sheer abundance (In RSE, the best HM slave is commonly believed to be Tropius, who is very rare.)
Pachirisu is one early on, due to their unusually high defenses to go with their high speed. It doesn't help that it's just seen by many as a Pikachu expy.
Phione is not popular, due to being a weaker version of Manaphy with worse stats, a worse movepool, and the inability to evolve into Manaphy. Many consider it to be "useless", as it fulfills next to nothing, gameplay or story-wise.
Sailor Kelvin can be found in the Master Rank of a few Super Contests, and it is highly likely that he will sabotage your chances at winning during the Performance round with his Buizel named Zippy and its Water Sport. And your day will be ruined even more if he picks the same judge as you.
Tier-Induced Scrappy: Luxray has a very popular design, but unfortunately, its stats and movepool just don't work together at all. That hasn't stopped it being used by quite a few players, though.
Most of the brand new Pokemon in this generation had good movepools, but mediocre to bad stats, with exceptions like the starters, Garchomp, Lucario and Staraptor (among a few others) notwithstanding. It seems that even-numbered Generations of Pokemon tend to have Pokemon with this problem, while odd-numberedGenerations have Pokemon with strong stats but shallow movepools.
Electivire might be one of the biggest example of this and Hype Backlash. Good Super Effective coverage, and pretty decent stats combined with its decent synergy with Gyarados. Sadly, the fact that its starting speed are pretty low for a sweeper of its kind(to clarify, Electivire is not a set up sweeper. A set up sweeper such as Gyarados and Lucario has a set up moves that gave them insane power and a way to bypass their low speed), and Motor Drive doesn't always guaranteed to activate, and the fact that its movepools doesn't include high powered STAB physical move means it wont do much damage without insane amount of efforts. Even during generation 5, Electivire is still a commonly bashed Pokemon.
Scrappy Mechanic: Stealth Rock. Holy shit. While entry hazards are essential to prevent the game from turning into a switchfest, there were two mistakes in Stealth Rock: making it a TM which everyone and their mother can learn (thankfully fixed in BW), and making it Rock-type. Rock type is pretty much the best offensive type in the game. Unlike Spikes or Toxic Spikes, nothing is immune to it, so the only way to get rid of it is the still rare, still weak, and still useless against Ghosts Rapid Spin. More than that, Flying types with their famous immunity to Spikes are weak to SR, meaning each time they enter the battlefield, they are hit for a quarter their health, along with anything Fire, Ice, or Bug. Any combination of two of those four types are hit for half. It defined the metagame so much, Pokemon analyses often go "Yadda yadda, good stats, diverse movepool, lots of resistances, all-around a solid Pokemon - no, wait. It's weak to Stealth Rock. Nope, don't bother with this useless piece of shit."
Not helped at all by the fact that it became available at the Celadon Department Store in HGSS, so it became extremely easy to dish it out onto all of those Pokémon that can learn it. That fix in Black and White wasn't necessarily limiting who could learn it, but rather removing it as a TM. It was then re-added as a Move Tutor move in Black and White 2 for 10 Green Shards, which made it a little harder to get than in Gen IV, but it's still a pain.
Not as much annoying but still annoying is the Pal Park limitation on migration - you can migrate only once per day. Of course, there are ways to effectively get around the limitation, but one should ask why they did implement that in first place. They learned from that mistake in HGSS, though.
Defog is one of the franchise's most hated HM's. It's used to remove fog in the field, which lowers the accuracy of every move used in battle, in what was viewed as a totally unnecessary and frustrating weather effect designed to waste a valuable move space on your Flying-type (only Flying-types can learn it) for the sake of having an even eight HM's. It's not a move that most players would want on any of their Pokémon, whether they play competitvely or otherwise. So you can choose to ignore Defog and grind your teeth out trying to get through the area with limited visibility and lowered accuracy, or you can waste a move slot on your flyer or a party slot on an HM slave.
Sidetracked By The Golden Saucer: As with every other pre-Gen V game in the series, there's a casino. And whenever that happens you end up spending hours on end trying to get the grand prize.
The elusive Feebas, a major cause of people getting sidetracked, is in the basement of Mt. Coronet, which also happens to be where Cyrus plans to unleash Palkia and/or Dialga on Sinnoh. Don't worry, Team Galactic will wait until after you're done hunting down a rare fish before they try to destroy the world.
The Underground is a minor version of this compared the casino. This place provides several useful items with some of them only being available from there. You tend to forget you were supposed to be level grinding for the badges when you were having a lucky streak of harvesting evolutionary stones, fossils if you're lucky, heart scales, and shards.
Surprise Difficulty: Because the original Sinnoh dex, well... sucked (see Character Select Forcing), a lot of the gym leaders don't actually use their preferred type for the majority of their team, making them harder to sweep with a single Pokémon that knows a Super Effective move.
The most notable examples are Volkner and Flint. For each one, half of the Pokémon on their teams don't follow their preferred typing. The electric trainer Volkner has an Octillery and an Ambipom, while Flint's got a Steelix, Drifblim, and Lopunny. Some of these Pokémon are put there to combat the common killers of their favorite types (Octillery beats Ground Pokémon and Steelix beats Rock Pokémon).
Another grueling example is Candice, an Ice-type leader who has a Psychic/Fighting-type Medicham on her team, somehow making the cut by knowing Ice Punch. A Psychic/Fighting-type is super-effective against all but one of Ice's common weaknesses (that being Fire), and can seriously trip up someone who thought that they'd sweep the Gym with a Fighting, Steel, or Rock-type.
Note that once Platinum revised the Sinnoh Dex, however, Candice, Volkner, and Flint fall much less into this trope than they did in Diamond and Pearl.
Tastes Like Diabetes: A few young child characters, such as the scientist's young daughter in Floaroma Town, or Tuber Chelsea on Route 213.
Chelsea:(after you defeat her) That cloud looks like giant ice cream! I bet it tastes yummy!
Tear Jerker: Barry's Heroic BSOD after failing to save Uxie from being kidnapped. "I have to get tougher. It's not just about being the strongest trainer anymore..."
Hearing Cyrus' past from his grandfather in Platinum. Especially considering what it's shaped him into.
They Changed It, Now It Sucks: In the European Platinum versions, you can't play the slot machines in the game corner, instead you can gather some coins from them once a day. This also means the only way to get the Explosion TM without trading or cheating is to hope the attendant randomly decides to give you one.
Unfortunately they didn't learn and in Pokemon Heartgold and Soulsilver you have to play a flip card game which has too much Random Number God for people to actually get stuff quickly and you can't just buy coins anymore. At least its an actual game, right?
They Just Didn't Care: Some of the contest types of moves introduced in this generation seem... odd choices, to say the least. Most glaring is Close Combat being classed as a Smart move, and Brave Bird being Cute. (Despite about 90% of Flying moves being Cool)
Jerkass Woobie: Volkner, though he's not much of a jerk, just tired of battling trainers that barely stand a chance.
Woolseyism: Looker is named Handsome in the Japanese release. Not only is it another way of saying handsome (i.e., one way to say someone is pretty is to say they're "quite a looker"), but it also ties in with his profession (he's a cop, as in, he looks for clues).