It has a few flaws that keep it from being truly great.
Filler arcs: A lot of anime have filler arcs that delay the main story and keep it running. I do not have a problem with that at all. however, these fillers are unlike anything I've ever seen. These filler arcs actually happen during a main story arc and are not even remotely canon with the series. The only filler arc I remember that never really did this was the Bount Arc.
Filler Arc stories: Some of these arcs are excellent (even when they interrupt a main story arc) but some are single episodes that don't even make sense. Some of the episodes were even stupid, such as when they did those Halloween themed episodes.
Continuing after the defeat of Aizen: Seriously, Aizen was an awesome villain and his defeat was perfect to conclude Bleach. After his defeat though, new enemies appeared and even before Aizen's defeat, I saw Bleach's quality decline, especially with the filler arcs.
Other than all that, it is a great anime that had great action and comedy mixed into one. It also had great openings and music in it and a great cast of characters.