An interesting addition, but overall a step-backwards
I'll go over the primary issues I have first:
1. Balancing issues: Capcom, for some reason, took a game that was nearly perfect in terms of balancing and made some very screwy decisions. Roster changes are varied, and many characters are vastly different from their likenesses in Super. Capcom very liberally nerfed many characters in the cast, a generally unpopular approach to balancing a game.
**As of this review, Capcom has admitted that there are some glaring balancing issues with this version, namely with Yun, and that they are considering yet ANOTHER version in 2012.
2. Roster additions: I feel like Capcom missed the mark. Yun and Yang are relatively interesting, but higher level players report that they're unbalanced. Aesthetically they're very similar even if they play very differently, which gives the appearance of redundancy (even if this is totally false). The addition of Oni and Evil Ryu is something I really dislike. These roster additions are bland, generic, and such fan-service it almost seems too lame to be true. I can remember ridiculing people in forums for asking for Evil Ryu, only to have him announced soon after, and just sort of having a "WTF" moment.
Oni is a fairly cool character with several unique moves (like an air dash). Evil Ryu is a rushdown character who hits harder, is similar to and just generally worse than Akuma.
3. New replay functionality: I've yet to delve into this, but you can track some of the top ranked players through the replay mode. This seems like a cool feature that can help players get some fresh ideas for their game if they feel like they're getting stale.
I feel like this version of the game is weaker than Super, which was probably the best iteration of SFIV so far. I'm hopeful that the 2012 update will do a better job at balancing and that Capcom graces us with a few new interesting characters to boot.