Nemesis was a new and exciting B.O.W - thanks to the inclusion of the parasite, it remained intelligent despite the physiological effects of the T-Virus - in tandem, the two things could co-exist and it worked great. When the parasites replaced the T-Virus altogether and you began fighting some weird creatures rather than zombies and B.O.W's, stuff that looked like monsters from "The Thing" rather than the standard assortment of enemies, it lost a lot of its charm. The T-Virus mutates the victim, but in a very specific fashion, the parasites used to include tentacles tops - I'd understand if it was the G-Virus causing all this crazy stuff because it was much more random and less benign than T, but... it's not - it's the parasites these days, at least in the games I played, and that's pretty plot-breaking.
