I've run two 4E campaigns that both fizzled out in the mid-Paragon's due to a combination of availability and, to be brutally honest, DM exhaustion. By the time we got to those kinds of levels, combats were taking an age to resolve and, while the players loved this aspect of the system, after a few years I found it very wearing. To be fair, that is all told about the only thing significantly wrong with 4E in my eyes. I made overtures to remedy the problem but always hit a brick wall of players having too much fun to want to change.
I regret not forcing the issue, though, because it slowed the pace of my homebrew to the point where the PC's impact on the campaign world was only just starting to be felt at the time the game went cold. This was a sharp lesson that I internalised at the time, but honestly, 5E plays so fast that it probably won't be an issue. We've played several sessions of AD&D in the meantime and after 4E, the pace of those sessions was almost dizzying!