Adolmaea was built up to be a great and powerful threat at the end of the first game, but turns out to be a pushover thanks to Cisna's sealing magic.
Grazel is unexpectedly easy in the first game but he turned out to be Not So Harmless in the second.
The Sun King in the first game, because Princess Cisna's spell holds back his power. You get to fight him without this assistance in the sequel, and, as a result, he lives up to his reputation as a formidable boss-fight.
Big-Lipped Alligator Moment: Leonard dancing with the princess on the water (just like Tidus and Yuna) comes out of nowhere, is never mentioned again, and it is unclear if one or the other is imagining it, given Cisna's costume change. Given his character, it's most likely all in Leonard's head.
Wraiths. They're Knight-sized and completely invulnerable to physical damage (unless you hit it with holy light) which means if you aren't a dedicated mage or have a mage in your party you are done for when you fight it. Adding to that it has an annoying scythe sweep attack that knocks you off your feet if you are not wearing heavy armor, a Life Drain spell which it makes liberal use of and, to make things even more infuriating, can summon five Succubi at once to aid it in battle.note See Goddamn Bats if you aren't familiar with the Succubi.
Spirit Lords (Efreet and those like him) are pretty tough too. They spam powerful sorcery, and summon enormous flocks of elemental fish to help them pound you into the ground as long as their summoning appendages are not broken. This doesn't seem so bad, but there's two high-end Superboss versions. And, to make the above one seem more painful, another one of those bosses mentioned is a Wraith too.
Fridge Logic: Every town has an Incorruptus mechanic even though there are only five known Incorrupti in the world and the one the Avatar gets near the end of the game is the prototype of the Arc Knights.
The Skeletons, basic humanoid enemies who hang out in the same place as the Wraith mentioned above. If you are underground or in a sufficiently dark place you can count on running into some of these. What makes them so annoying is that they are invulnerable to physical damage. You have to hit them five or six times before they collapse and give you a maybe 15 second long opportunity to clobber them after which they reform and the game start anew. The only upside is that they can be one-hit-killed with holy light once their soul is exposed.
The Succubi, who ALSO happen to reside in the same area, are not much better. They have the Wraith's 'being completely immune to physical damage until hit with Holy Light' gimmick, and they add onto that with something that makes them even more annoying than the Skeletons: their scythe attacks can paralyze you and they have a MP leech attack.
It's Easy, So It Sucks!: The first game is fairly easy unless you're new to JRPGs. Origins is EXTREMELY easy and very very simplistic (Rushing at the enemy will generally lead to victory).
It's Hard, So It Sucks!: If you expect the second game to be as easy as the first and it will rip out your spine and hand it to you. The second game is the counter punch to the arguments about the first game: Stronger enemies, weaker Knights, and a leveling curve roughly the shape of the Matterhorn. Oh, but at least they sped up the turn gauge.
Moment of Awesome: The first time a character is shown transforming into a knight.
Caesar after he defeats Belecitane
Belecitane: "But how...? My Gigas...!" Caesar: "How? I just busted your ass, that's how! Any questions?"
Sequel Difficulty Drop: The first game isn't exactly difficult, but Origins is very very easy, and even for the more difficult bosses, the strategy of "add more + to your equipment, and go into Power Ranger mode before you die" will get you through them.
Sequel Difficulty Spike: The story mode in the sequel is much harder than in the first game. Monsters are more numerous (including giant mooks), have more HP, hit harder, and your final line of defense, the Knights, are much weaker and there are several stretches near the end of the game that leave you without any Knights at all.
So Okay, It's Average: The first game had some pre-release anticipation as a possible Killer App for the PS3 due to the “You get to drive a Humongous Mecha” angle. But the end result was a bog-standard JRPG on a platform crowded with them.
Spiritual Successor: To Level-5's unreleasedTrue Fantasy Live Online, from which it (probably) borrowed its character creation system.
Strawman Has a Point: Sarvain is set up to be Obviously Evil right from his introduction, yet when he assumes control over Balandor following King Valtos' death and starts issuing orders, he actually comes across as the smartest and most practical man in the room, especially compared to Cyrus, who's first reaction is to lobby for an attack on Faria and then storm off in a huff when Sarvain shoots him down.
Some people who play this the first time around regard the Sequential Boss segment in the Bunker Lode Caverns (Belctiane, two Gigas, the Dragon Matriarch, and Sargatanas) as this.
The Netherwyrm in the second game can be pretty tough if you didn't level up the Dragon Knight enough or stock up on HP and MP replenishing items in Ceasar's inventory back in the first game, or if you're starting straight from the second game (where you at least have contrived MP regen to compensate, and even that's usually not enough). And this is after the Wyvern Rex fight, so if you fail here and abandon the battle, you'll have to do this all over again.
The two Wyverns you have to face in Van Haven Wastes are tough without being able to rely on the Knights. There's even an Ominous Save Point nearby because it knows you'll need it.
Past Pyredaemos is pretty tough too, for the same reason as the Van Haven Wyverns. In addition, you've got Cyrus as a guest party member; his death is a Game Over. For many players, when everything that happened before then wasn't enough to make them quit, this fight was their breaking point. Apparently, this was supposed to be the big hint that you should be grinding away online to get better equipment and skills.
There's also a particularly nasty Boss Rush after this. First you face Past Dragias. He's sort of tough, but not all that bad to defeat if you've been managing your movesets well and as long as you keep Cyrus in good health, but right after that, you will face the Sun King with Yulie and Caesar solo. All you need to know is that you can't defeat him and you need to survive long enough. He hits like a truck. Lose either Yulie and Caesar and it's game over. Survive long enough, and you have to face him one on one with the White Knight. All you have to know is that there's no chance to save in between. Lose once, and you have to start all over again.
Also note that pressing "Retry" will put you back at the beginning of Balandor's Secret Passage. That's right, if you didn't go and save at that logic stone for whatever reason, prepare to go through a dungeon that contains the aforementioned Succubus and Skeleton and Wraith.
Brimmflamme is on this for a whole different reason. First he's huge so you need to put down some chains to seal it. So there's a small mini dungeon path to go through first. Not that hard, but you have to fight Brimmflamme's head in the middle of this and it isn't a cakewalk. Then the actual Brimmflamme fight. A standard battle, access to knights. Then, in the middle of the fight, Leonardnote who's been laid up with Knight Fever for half the game and is probably severely underlevelled joins the fight. The fight is mandatory to have Leonard/White Knight. Worst part? The Black Knight decides to join in. If you lose, prepare to do all of that again. If Past Pyredaemos wasn't your breaking point, this one sure as hell can be.
That One Level: Frass Chasm. A huge area that's tough to navigate because of the dandelions you need to use to get around.
That One Sidequest: A large majority of the errands, but special mention has to go to the Flemron 108 quest. You have to get it from a monster who only spawns in one area in the game, and the drop rate for said item is near zero. Be prepared for days of farming.
They Wasted a Perfectly Good Plot: Be warned. Origins for the PSP may take place ten millenia before the first game, but you will NOT be running around as the people who originally sealed the Knights away and helped Mureas fight Madoras. In fact, aside from a few encounters with the Knights and the Gigantes, the game goes on a complete tangent and tells the story of Marhk, a kingdom wiped out by Yshrenia and completely forgotten by the events of the main series. Your avatar is tasked with stopping the apocalypse, the cause of which is completely separate from the Dogma War.
Really, the main games really leave a lot of player wanting for better plot points, always teasing potential ones that are simply never realized.
