I find it interesting that most of the sequels you mention are part of longer franchises.
Its strange how with video games sequels are almost mandatory, I mean it sells copies right?
Where as in the movie industry (at least from my perspective) the real gems are films that don't have sequels because they don't need them.
e.g.
Donnie Darko
Fight Club
Shawshank Redemption
Primer
In my opion, most games with "2" suck, but if the series makes it to "3", they start to get the gist of what people want from their games. Heres some examples:
UT(good) > UT2k3(suck) > UT2k4(good)
Devil May Cry > DMC2(suck) > DMC3(good - well better I guess, not as good as 4 imo)
Red faction(good) > Red faction 2(suck)
Deus Ex(good) > Deus Ex 2(suck)
Metal Gear Solid(good) > MGS2(suck) > MGS3(good - well at least it was snake again, but I actually stopped playing half way through because I really hated the way the story was being told)
Halo is a little different, as the singleplayer got worse and the multiplayer got better.
Halo(good) > Halo 2(worse single better multi) > Halo 3 (terrible single and good multi)
I havn't played these sequels, but these are differentbecause a diferent team gets the licence to call their game "2" which really pisses me off, cuz you wouldnt watch a sequel to a film with a different writer/director/actors right?
e.g.
Operation Flashpoint > Operation Flashpoint 2
F.E.A.R. > F.E.A.R.2 (originally, then FEAR2 fell through, and the real team got it back)
CoD Even numbers > CoD odd numbers
Or what about sequels that have nothing to do with the previous game?
Final Fantasy games
Far Cry > Far Cry 2
Gotta say as far as Diablo/Diablo 2 and Warcraft 1/2/3, a progression that just got better I think, let's hope Starcraft 2 follows the trend ^^