Accidental Innuendo: A construction worker in the Pyrite Hotel says "Apparently some rich fellow ordered the colosseum built. I'm amazingly stiff."
Alternative Character Interpretation: Wes, the Silent Protagonist. He spends almost all of the game frowning, save for his introduction, where he grins wickedly upon stealing the Snag Machine and wrecking Team Snagem's hideout. His motivation and purpose prior to meeting Rui are complete mysteries.
The last postgame Shadow Pokémon you have to snag is... a Lv 20 Togetic. That's considerably lower-level than the Pokémon you started the game with, and Togetic isn't a particularly strong Pokémon at that. The biggest danger in snagging is it is not accidentally knocking it out. (Which is very easy to do, as you're expected to have a Lv 65-70 team at that point)
Inity, the Mt Battle Master in Battle Mode Doubles. His team isn't terrible, but coming after battles 98 and 99, which have Groudon and Kyogre respectively, he's a total joke. Contrast that to Infin, the end boss of Singles Mt Battle, who has a Kyogre.
Awesome Music: Due to the increased space allowed by being on a console instead of a handheld, the soundtrack is much more grandiose than in any other Pokémon game.
The final boss theme does an excellent job of highlighting the difficult battle against Evice.
Best Boss Ever: The final battle against Evice is one of the hardest bosses in the series, but unlike some bosses, the fight contains legitimate strategies that actually has you think of your feet. The music is amazing, and highlights the end of the adventure and difficult fight ahead and the background and tension for the battle is awesome.
In series canon the entire game and its sequel are one, as they have never been mentioned in any other games and both games have very noticeable quirks and are set in a World of Ham.
In the context of the game, the encounter with Cipher Peon Mirakle B. He's a Loony Fan of Miror B. who wears a uniform modified to look like him and randomly shows up to battle you if you go all the way through Pyrite Cave to Miror B.'s hideout after beating Dakim (but before beating theFinal Boss). Beating him gets you nothing, and unlike with Miror B., you don't even get an easy way out of the area.
Breather Boss: After several tough rematches with the four Cipher Admins and before the battle with the head of Cipher, Gonzap is actually not that tough at all. He even has some Artificial Stupidity, spamming Earthquake on the three of his Pokémon that have it, even though every time he does it, it knocks big chunks out of whoever else he has out at the time. Using Flygon makes it even funnier, since Levitate makes it immune to ground moves. His Shadow Skarmory can be a pain to catch, though. (Unless you swap-duplicate the Master Ball, then it's a cinch.)
Cult Classic: One of the biggest for the series. Colosseum (and to a lesser extent Gale of Darkness) remains very popular with long-time Pokémon fans, to the point where its fandom is almost as big as one of the main series handheld games.
May also cross over into Harsher in Hindsight, depending on how you view the issue. One of the commercials for the game depicted an angry mob, with a man prominently shouting "We will get our Pokémon back!", which becomes almost prophetic in light of the outrage in the announcement that the Gen 8 games wouldn't get support for Pokémon not in the region's PokéDex.
Just Here for Godzilla: 99.9% of all discussion of this game is focused on its Story Mode, with Battle Mode (which is essentially Stadium 3) being forgotten despite the fact it was likely meant to be the main point of the game. The developers seem to have been aware of this, as the sequel was made with story mode in mind and only has a tiny, tacked-on Battle Mode.
Moral Event Horizon: Dr. Ein crosses this just by inventing the Shadow Pokémon system (without a shred of remorse, and actually with a desire to improve it), and Evice just by leading the whole operation. Particularly Evice, since he oversaw all these despicable actions while posing as the friendly mayor of Phenac City.
Most Annoying Sound: The alarm in the Shadow Pokémon Lab. It doesn't really do anything except annoy you, since the areas you've cleared out stay cleared. There's also no way to avoid it, as it's a scripted event that happens after defeating a required enemy. Worst of all is that it replaces the incredibly catchy background music.
Rescued from the Scrappy Heap: Due to a lack of wild Pokémon and the extremely limited choices available to the player in this game (a total of 52 at the very end of the game) many Pokémon who are seldom used due to lack of usefulness in competitive play get the opportunity to shine here. One reviewer said it best in that this game forces you to use Pokémon you would usually never even think twice about just because of how few choices you get, especially early in the game.
The move Shadow Rush inflicts recoil damage, which can deny you a Snag if the computer used it after you whittled a Shadow Pokémon down to low health.
Hyper Mode forces you to forfeit a turn to dispel it, which can be frustrating if the Shadow Pokémon that contracted it is at low health and gets KO'd because of it.
All battles being double battles. As double battles aren't as streamlined as they would be in later games, this makes matches drag on much longer than they should.
Multiplayer requires GBA cartridges, there's no rental Pokémon roster, and Quick Battle is only one player.
The fact that shiny Shadow Pokémon probably wont remain shiny once caught because the mechanics behind shininess. It was actually an oversight relating to the personality value and fixed in the sequel with a shiny lock for Shadow Pokémon. Non-Shadow Pokémon actually retain regular shiny chances like in other Generation 3 Games.
Special Effect Failure: Despite Jumpluff's body being made up of four spheres, it's shadow is four squares. The sequel fixed this.
Spoiled by the Format: Most of the trainers, most notably two towards the end, use Pokémon that were technically available to you when the game was released. So, you'll see evolutions or pre- evolutions of Shadow Pokémon all over the place before you can snag them, sometimes way before. You'll see Spinarak all over the place, including one in the first city, but you get to snag Ariados just about halfway through. Colossseum uses Pokémon that are available in Pokémon Ruby and Sapphire plus all the Gen II ones, including ones that evolve from Gen I Pokémon that are unavailable in Ruby and Sapphire.
Miror B. and his Ludicolo. The only weakness they have that you have access to at this point in the game is Flying, and only through a near-purified Noctowl/Swablu's Fly. They all have Rain Dish and know Rain Dance plus a group of restoring moves (Absorb, Leech Seed and Mega Drain) which make the fight take a long time even with Fly, since it's a two-turn attack. This battle a true challenge of patience.
Dakim has three Pokémon with Earthquake and Protect, and they will alternate between them so one hits the entire field with Earthquake and the other doesn't take anything by Protecting. Earthquake also hits his Shadow Entei super effectively and it doesn't have Protect, so if you're not careful he'll knock it out and prevent you from catching it.
Venus is among the most tedious bosses to fight in the whole game. Her entire team except her Shadow Suicune knows Attract, which makes Pokémon of the opposite gender unable to attack 50% of the time. Her Delcatty hits surprisingly fast and hard, and on top of infatuation, her Vileplume can paralyze your Pokémon with Stun Spore, making them only able to attack 20% of the time if affected by both. To make matters worse, her Pokémon have a sudden level spike from the Pokémon you fought previously by about 5 levels, which makes the fight more difficult to those who haven't grinded in preparation.
Ein has three Confuse Ray users and two Toxic users to try and hax you out of turns and stall you out of HP with the passive Poison damage. His Lanturn and Huntail know Rain Dance to boost their Water attacks (and give Huntail a Speed boost since it has Swift Swim) and give Raikou and Lanturn 100% accurate Thunders. Like Venus, his team is also about 5 levels higher than the next strongest trainer you fight, so he may also overpower you by level advantage if you haven't been grinding.
Evice is very challenging even by Final Boss standards, especially if you allow Slowking and Slaking out together. Slowking WILL use Skill Swap and Slaking will use its high base stats and freedom from Truant to destroy your team. His others are no slouch either, since they all know moves that will boost their Attack. Examples are his Salamence with Dragon Dance and Double Edge and his Scizor with Baton Pass so it can pass Swords Dance boosts to a teammate, as well as Silver Wind which hits both of your starter Pokémon. Besides from your Espeon your Pokémon are highly unlikely to outspeed his Salamence and Slaking, and there is a massive level gap between him and Nascour. Due to his unique strategies and strong Pokémon he is considered the hardest final boss in the series, alongside Ghetsis and Cynthia.
The abandoned mine hideout of Miror B. It's a maze of tunnels and bridges and caverns and they all look alike. The random trainers scattered throughout will have you jumping for joy when you stumble onto the heal machine before the boss battle. Although you get a cut scene skip to Duking's house after battling Miror B, you have to find your way in and out on your own when you go after Mirakle B. And if you thought you could use an Escape Rope like in the handheld games... you can't in this game.
The Shadow Pokémon Lab has a very linear layout but has two major annoyances. First there is only one PC where you can save with, located at the beginning and no heal machine at all. The only way to heal your Pokemon without exiting the Lab outright is by very tediously depositing your party into the PC and withdrawing them afterward. Then there is the other problem of Cipher Peons dropping in (literally) and ambushing you when your Pokémon are weak.
Realgam Tower has a lot of tough battles to get through, several of them with Cipher admins, and the level spike of Nascour and Evice means you have to grind to stand any chance of beating them. Nascour is in the 50s, and Evice at 60-61. Plus, you can't save after each battle in the final part, meaning you have to deliberately lose to one of the normal trainers and then save to secure snagged Shadow Pokémon (fortunately you're healed after each battle). However, Nascour and Evice are both much higher than anyone else you've faced, and you likely won't be strong enough the first time you fight them.
The Plusle that Duking gives you is a Joke Character in-game. However, if the developers had made Minun obtainable, as Dummied Out text from Nett suggests they had at one point planned to, it could have been a rather good option. The abilities of Plusle and Minun, Plus and Minus respectively, raise the Special Attack of both Pokémon if they are allied with each other. Considering this game is all Double Battles, it's rather frustrating that they didn't end up including Minun.
There are plenty of fans who would've preferred the menacing and much more interesting Nascour to have been the true head of Cipher to the somewhat flat Evice who's reveal was somewhat shoehorned in.
For how heavily they were featured in Wes's backstory and how prominent they were in the opening scenes, Team Snagem really isn't featured in the game all that much until post-game. In fact they aren't even mentioned once you get more involved in Cipher's activities.
Uncanny Valley: The way Rui's eye bulge to the sides makes her face quite unsettling.
Underused Game Mechanic: Theoretically, Snagging allows the player to capture any Pokemon. In the game proper, though, Rui prevents the player from trying it on non-Shadow Pokemon, and there are only around 50 Shadow Pokemon. While there's a reason why the developer didn't allow players (especially young ones) to catch non-Shadow Pokemon, it would have provided an interesting dynamic if players had the freedom to Snag whatever they wanted, possibly with a reward to Snagging only Shadow Pokemon. They even could have programmed it so Snagged non-Shadow Pokemon can't be traded over to the main series games.
Viewer Gender Confusion: Nascour is easy to mistake for a woman, having long flowing hair and a sizable bustline due to being muscular, in a game where female bodybuilders are rather common as trainers and NPCs.