I define a game as 'bad' if I literally cannot bring myself to continue to play the game. Most of the empirically bad games I've played I ended up having a lot of fun with - like Trespasser, Ultima 9 and Sonic 2006. Empirically bad games are usually so poorly-made and buggy that at least a slight amount of entertainment can be obtained from the spectacle; or at least from the schadenfreude of knowing that someone's career came to a crashing halt because of it.
Generally the games that I find truly bad are, on paper, extremely good: well-polished, well-designed, executed flawlessly and ultimately represent an excellent business decision for the producer. The quality of the game is the problem: they're so polished they seem mechanical rather than artistic, so well-designed you can't even count the number of people on the design committee, and such a good business decision that it appeals to the broadest audience possible, reducing the entire affair to an inoffensive and tepid ordeal. They have no soul. There is no intellectual dialog between the consumer and the author.
When something is a labor of love there are always going to be issues; sometimes serious, crippling technical issues. But the cure is worse than the disease.
Modern examples:
EA's entire library.
Heavenly Sword.
Resistance 1
Halo 2, 3.
Super Smash Bros. Brawl.
New Super Mario Bros. (DS)
Every WoW clone
God of War 2
Gears of War 2
Fable 2