I'm not very good at fighting games. Memorizing combos just isn't for me and the kind of dedication and experience needed to be even slightly competitive is pretty daunting. It also takes a really specific kind of dexterity that you really can't prepare for by doing anything else.
What I'm saying is that I get absolutely owned online in Street Fighter IV unless I'm playing against someone who doesn't even know more than one of their character's special moves. However, you don't need to be a fighting game semi-pro to enjoy Street Fighter IV.
Only barely know how to play Ryu and Ken? Great! Don't even know how to play Ryu or Ken at all? That's fine, too! There's a Trials mode in the game that essentially teaches you the mechanics of playing each character. The only problem with it is that getting the timing on combos is something they don't tell you anything about how to do. They just tell you some moves to do in succession and leave you to go at it. As Jeff Gerstmann mentioned in his review, this game won't teach you theory/strategy. So while you may know how to string some 5 hit combos together, you won't really know when to do a specific one unless you do some research.
The other single-player options are Survival Mode, Time Attack Mode, and Arcade Mode. All of those are pretty self-explanatory as well as enjoyable. Arcade Mode's narrative didn't do much for me, but I doubt many people play Street Fighter IV for the story. I found Arcade Mode on Medium difficulty to be just challenging enough, except for the final boss, Seth. After you beat him once, he decides to really get really real and the difficulty ramps up quite a bit. It can be pretty frustrating.
The graphics in the game leave very little to complain about. They're not really technically astounding as much as they are very appropriate for the game. Everything looks good. It's hard to say much more about it than that. Everything seems to animate well and the style really just works.
I can't speak for die-hard, skilled fighting game fans, but I think there's plenty here for a novice to enjoy.