One of the main things I've heard people complain about is the multiple simultaneous fights, which started at the end of the Soul Society arc with Ichigo v. Byakuya, Soi Fon v. Yoruichi, Kenpachi v. Komamura + Tosen and Ukitake + Kyoraku v. Yamamoto all happening at the same time and the latter two fights are never even resolved.
Personally, I think the main problem is that Kubo made too many characters so he ended up spreading himself too thin with making all of them interesting. For example, I don't think that there needed to be so many Espadas and Vizards. Only Espadas 1-4 did anything worthwhile (Grimmjow was just a device for giving Ichigo some balls again and Nnoitora got curbstomped as soon as a competent fighter took him on) and Shinji is the only Vizard who does something worth a damn. Whilst interesting villains are necessary, Kubo already had three, he did not need to add another 10. When I stopped reading, there were so many loose ends left hanging that in no way looked as if they were going to be tied up and, from what I've seen of recent chapters, they still haven't been with the exception of one (what happened to Kensei, which to be honest I couldn't care less about anyway).
I think the point about Ichigo becoming too powerful is valid to an extent but that's just a problem with the genre.
For my sins, I intend to get caught up on Bleach and will read the final arc (although I'll probably just let it finish and read it all in one go). For all of its shortcomings, I still hold a soft spot for it since it's the series that got me into anime.