I think you're right about 2K finding little reason or point behind concentrating on significant current generation upgrades. But for most, again, I think the question goes well beyond, "What's the problem with having the same graphics but improved gameplay?"
When aesthetics feel largely untouched (actual drawn graphics, atmosphere, dead ball animations, presentation, assorted movements, "little things"), that tends to leave all of the improvement focus on the actual gameplay - and gameplay improvement is good. But now, the question for many will seem to be, "Is the gameplay improvement enough to warrant spending $60 on a brand new game?" Whereas in previous years, there tended to be a lot coming at gamers from various angles of the new 2K game itself: new gameplay improvements, significant NBA mode upgrades, new NBA-based modes introduced (Classics, MyPlayer, NBA-led USA Dream Team, Dunk Contest, etc.), heavy presentation alterations and yes, even improved graphics some years. In the case of the latter, there was a little more comfort in upgrading, because there seemed to be a litany of changes from many different angles.
Essentially, the wonder at this super early stage (a stage at which, by last year we already had two legitimate video developer insights on specific visual evidence of new gameplay, animations, and dribble mechanics), is whether the gaming experience is going to be different enough. After the freshness of the smooth dribbling system and paint contact wears off and becomes second nature, is every other portion of the game going to tell me I'm still playing 2K13?
Lastly, $60 is a fair chunk of change regardless of circumstance but again, I understand your logic and reasoning behind the worth gained from being able to use your product for an entire year. It's a fair point. However, I'd wonder how many folks ceased playing 2K13 six month deep or prior. For me, 2K13 had a shorter lifespan in my house than any other 2K over the past ten years. So again, the question becomes, if 2K13 lost me so soon - will 2K14 provide enough to make the game feel fresh from many angles? Will the on-court differences be enough? I do not have those answers yet, but the early return seems to raise at least a little bit of doubt, and I do not blame others for feeling the same thus far.