In regards to the III series, I thought the first two were intriguing, but not spectacular. HOWEVER, 3rd Strike just made everything click. 3rd Strike's my favorite Street Fighter because of its fluid gameplay, slick presentation, and variety of opponents and level paths. (The only thing it's missing are the transforming levels from III: The Second Impact). Street Fighter Alpha 3 is my second favorite, and then probably Super Street Fighter 2.
Street Fighter IV is a strange animal. It looks like it'll be OK, but it almost looks like "Street Fighter: Fan Tribute" or something. It's supposed to take place between II and III, yet has a number of Alpha Series characters (miraculously back from the dead or not aged), and basically reintroduces every character from the Street Fighter II era (excluding maybe T. Hawk?). I can't believe they want to say this makes sense continuity-wise. But, I won't worry about that. I just think it doesn't make sense to disregard Street Fighter III and try to make what is, above everything else it seems, another expansion/redux of the redone-to-death Street Fighter II.
There was no need to bring back so many old characters...except maybe to cover up for the average nature of most of 4's small new cast. (Even the EX series fighters were more inspired than the new fighters of IV).
I think the best thing would have been to build off of 3rd Strike's cast: Keep Ryu, Ken, Chun-Li, and Akuma from the old II series; add the story-important/definitive characters of III which are Alex, Sean, Ibuki, Elena, and Urien; then, if need be, add a few more classic characters that fit in the time frame, and finally STACK IT with fresh faces to make it a new experience.
Despite the ups and downs of the III series, it tried to bring a new, fresh feel to Street Fighter, just as II did to #1, and the Alpha series did to II. As it is, I don't think IV is being bold enough to define itself.