I loved Timesplitters 2. I loved the single player, I loved the multiplayer, I loved the challenges - it is, to me, the absolute pinnacle of the console FPS. And when it comes down to it, most of the reason I loved it was the subtlety and simplicity of the whole package.
Timesplitters 2 has no plot. It has a background of sorts, if you read the manual and don't skip cutscenes. But when playing the game it has all the narrative complexity of the average beat 'em up. Alien "Timesplitters" bad, humans good, Cortez and Hart trying to save world by entering a range of brilliantly pastiched scenarios and completing some pretty arbitrary objectives. I can't really emphasise enough the genius of the humour in TS2, it was wonderfully understated - the parodies of different film genres and characters down to the music choices.
Timesplitters 3 by contrast tried to have plot. It tried to have comedy, turning Cortez from a mere mute Vin Diesel lookalike to a camp jokey moron. The deliberately cheesy dialogue went past being genuinely funny and into the region where you expect there to be a neon light mounted on your TV with "LAUGH NOW" flashing on it whenever a bad joke was made.
Game design suffered too: the level design appeared bland, co-op was utterly crocked (it teleports the second player around at the whim of the first player - with a friend we found P2 could get jumped from room to room by P1 walking along and back along a short corridor) and some twat decided that what it really needed was vehicle sections. No FPS needs vehicle sections, but somewhere along the lines it was decided that all FPSes should and so now they do, TS3 included.
Essentially the difference between TS2 and TS3 is similar to the difference between Worms 1 and Worms 2. Both Worms 1 and TS2 represent a more stripped down, simplified experience - both reward taking some time to learn some actual skill but the options aren't dazzling, what those games do they polish very well. Worms 2 and TS3 are both bloated by the success of the previous incarnations, cramming in more features, more weapons and brasher jokes for the sake of cheap laughs.