Well Deus Ex 2 isn't a terrible game per se... it had a lot of good things going for it that put it ahead of other shooters of the time, an interesting aesthetic, tight console controls(for the console anyway it was looser than a village whore on the PC), decent integration of RPG elements, dialogue trees, and Deus Ex's high standard of level design... but it is not in anyway shape or form a worthy successor to the original...
the setting and story we're too forced and far removed, part of what made Deus Ex great was it's grounding in reality, you know you go down an alley and hop up on some boxes to break into an apartment, you know that's all stuff we've seen before and gives your actions a greater impact than say breaking into a police hangar in a skyscraper to hijack a helicopter... the same with the story...
another problem is the RPG elements... it's dumbed down... there is little more you can say about it, the original Deus Ex gave you the ability to level your character in any fashion, letting you live and work with any aug or upgrade you happen to pick with no wrong answers... from being an under water platformer to a sneaky medic... IW didn't allow for this, you could either pick from being the under water guy or being the platformer... sure it helped make the game easier but it also took away a lot of the fun of developing your own personal character...
and finally is the combat... it was... too simple and streamlined and had very little RPG elements integrated... you just pointed at something you wanted dead and held down the trigger until it stopped moving(or started having a seizure depending on how the physics engine is feeling) there was little strategy to it beyond the few upgrades that gave the gun a situational characteristic but that felt tacked on... but simple doesn't always mean easy because the stupid enemy AI and balancing... having an armored enemy that can take 3 sniper bullets to the fore head before they died does not a fun game make...
also the inventory system and general lack of item management... I liked the one in Deus Ex because it made the player really think about what he needs or doesn't, you know a rocket launcher takes up the space of a rocket launcher, but in IW it's the same size as a candy bar so you can just pick up a whole bunch of guns and stack 20 medkits for later destroying the balance... though I'm honestly exaggerating a bit it worked pretty ok mostly because you didn't have a monumental amount of space... but it still bothered me for some reason...
I said before that the level design was good... well it is for the most part but sometimes it just becomes way too linear, especially near then end... now there's nothing wrong with linearity but there is a problem with it in this game... it sometimes makes properly stealthing your way through a level damn near impossible and forcing you into combat that feels like a punishment, the ending being particularly bad when they're rolling out 6 over powered armored enemies that will destroy you because the only path through the level is directly through their legs and you've been like a neglectful parent to you're guns because you wanted to be the stealth guy...<.<
Finally, Universal Ammo is an idiotic idea...<.<
Also I won't be complaining about the story since, come on, what did they have to work with... Deus Ex had 4 completely different endings and they tried they're best without picking a single one...<.<