C&C: tiberian sun.
I absolutely loved the original Command & conquer (I think I played through both campaigns 2 or 3 times). Red alert clearly improved on the formula, but I missed things like the flame tank, the obelisk of light and that ion storm the GDI had. Basically, I wanted tiberian sun (which was teased at the end of C&C*) to be a mix of both.
It was anything but that. The perspective was altered a bit, meaning there was less on screen (and less visible, but that could be due to the theming as well). I hated that base updating mechanic. There were no naval units. The B-actors in the cutscenes addressed someone else rather than you (who the hell thought THAT was a good idea?!). And most of all: some units were plain broken. Sure, the mammoth tank was always strong...but the bottom of this game was when I cleared the entire last level with just a mammoth tank and a repair vehicle I had from taking over an enemy vehicle building plant. This game had the doubtful honor of being the first game I ever returned to the store. I got less than half of my money back, but even that didn't concern me.
...but my first thought was UT3. The creators did some serious damage control after release, but before the black edition it was one horrible mess. The UI was a disaster (menu's had barely any items and required confirming so often that simply starting a game became a chore). It had barely any levels. The vehicle combat was plain not fun. And while epic wasn't known for their great level design, there were hardly any levels at launch either. More so: the iconic wildly different styles were gone too (there were about 4 different themes in the entire game, starring all a sort of black or grey). And for some reason, someone thought it was a good idea to remove the tournament from the story (mind you: "UT" stands for Unreal Tournament) in favor of a vendetta "story" that would even make the lousiest fanfic writer blush in shame.
People movement was a different thing. Granted: I had played UT2004 for years, and everyone agreed that the movement system (with all the dodgejumping antics) had too much of a learning curve to be enjoyable by new players. Epic listened a bit TOO much: Aside from a straight dodge you could do nothing, which meant that hardcore players always felt like they were moving through mud.
Oh, and probably the worst of all: finding people online was close to impossible. In UT2004, you could add favorite servers and search for players based on wildcards (eg: clan tags). At launch, you couldn't do either in UT3, thus forcing hardcore FPS players together with newbies, which ruined the fun for everyone.
Granted: on hindsight the game wasn't as bad rather than it suffering from overhyped expectations (and while my personal favorite gametype - bombing run - wasn't there, it had the best CTF maps and mechanics ever. Unfortunately, the only crowded servers I ever found were filled with snipers on CTF-Face...and that got boring after the zillion'th match).
*and with that probably had around 5 years head start on what's currently known as a 'teaser'.
