SLP2000: Blizzard is not a big publisher in terms of the amount of games they own. They onlu have Diablo 1 & 2 + expansion, Starcraft + expansion, Warcradt 1 & 2 plus expansion, The Lost Vikings 1 + 2.
That's possibly 7 items in GOG store (or 8 if you caount Lost Vikings as a separate item), so that doesn't make Blizzard a big publisher.
SEGA - what PC games they own? II was sure they own mailny console games, and GOG isn't interested in those atm.
THQ - same question, PC games arn't main part of their catalogue, and most of them are too new.
Blizzard also made Blackthorne, which is a great platform game.
SEGA published the Total War games, as well as Championship Manager (now Football Manager), and more recently Alpha Protocol (probably others too).
THQ published the Dawn of War games, Company of Heroes, Saints Row, Stalker, Supreme Commander, Titan Quest and more.
(info from Mobygames, apologies if there are any errors).
GOG has hinted in the past that we might get emulated games here, which could include console games or perhaps Amiga games.
So whilst these are smaller publishers, they still have some games to offer, and they might be considerations for the two publishers arriving in the autumn.