Where to start?
SimCity.. 1 = awesome, 2 = fab, 3 = bareable, 4 = utter poo.
C&C.. post Yuri's = pants.
CounterStrike.. <=1.3, bar cheats, and one well known exploit, was awesome. 1.4 was so garbage Valve 'unnerfed' it within a month but it was still a shadow of it's former self. 1.6 (specifically the introduction of the shield) = utter, utter shambles. Source.. well, they removed the shield but the only thing it has similar to 1.x is the names of items and title, other than that it's a compeltely different game.
Battlefield.. the players lost it for this game (for me.. ) too many spawn huggers, nobs who kill you because you got to the vehicle first etc. etc. etc. BF2 brought in ranks.. utter crap system to have on a game that is not an RPG.
All MMO's .. new Content gets far to complex, takes up far too much time to get the items absolutely required for PvP (DAoC for example.. to even bother with PvP you *must* have a full squad with the perfect template fitted on all toons.. an untterly insane amount of grinding with 50+ groups required for months to achieve this.) New content also piles on the pressure for the hardware, see Eve-Online for an example. Some insanely rare items making owners practically invincible.. see EQ.. watching a lone paladin pwn 50 players because he has 'the' hammer.. see DAoC.. Scouts with 'the' cloak able to take on 10+ people.. a scout ffs.
Doom 1, 2 awesome.. alleged industry firsts, but turned into graphics engine display in the 3rd.
UT.. 1 awesome. Real groundbreaker, stole the thunder from Quake3 (which is also an awesome game) UT2k3.. dumbed down, pretty graphics which all get turned off by serious players and covered with dayglo skins in competitions. UT2k4 dumbed down further and more 'worthless' graphics. UT2007(8?) looks to be a Doom3 incarnate.
The list goes on