You know when I first put the game in I was a little disappointed. I thought I would immediated see and feel the differences and upgrades but my initial reaction was that not much has changed. The more I play the more I notice and like about the game. The offense movement is night and day. Without calling a play you have players moving on offense trying to get open, making cuts when necessary (like when a double team occurs). Your teammates just seem smarter on both ends of the court.
The animations are also improved especially when contact is made at the rim. Overall more realistic. Again its something that doesn't stand out at first but the more you play the more you notice.
Lastly the control and feel. This is perhaps what I was most disappointed about when I first played. I got on offense, started dribbling, tried to drive and explore my options and I thought "I thought this was suppose to be a little different and more responsive". I wasn't feeling it at first. Again the more I played the more I noticed the differences from 2k14. Players feel a bit more free. They don't suction to players quite as much and I have noticed very few, if any, canned animations on offense. Dribbling feels a bit better but mostly the same.
The big thing, as you mentioned, is that the game is still 2k. If you are a 2k fan then you are going to like/love 2k15. The developers aren't reinventing their image here. For me my 2k side is loving this game the more I play.
However their is another side of me that wants something new and different. I've been playing 2k every year since 2k2. I love the series but I also want something to compliment 2k as a change of pace. I'm hoping Live will be that this year. It doesn't have to be better than 2k, it just needs to be a solid NBA game that plays well.