I love my L900. It's a fantastic piece of hardware. Saying that, I have talked about a few issues I've had that seem rather widespread after searching the Nokia Discussions board and also after speaking with Nokia directly.
The vibrate feature on the phone produces an increasingly annoying rattle. Mine started off very slightly but has gotten worse. This is a hardware issue - it's either a camera mechanism or the SIM card mechanism.
The screen is not as fluid and responsive as you may be used to. This seems to really be a difference in manufacturers. We've had 3 Samsung Windows Phones in our house and never noticed any screen lag whatsoever. This will be more apparent when playing games like Wordament, etc.
The back button on the bottom of the phone is horrible! When trying to back out of a game or movie, it will CONSTANTLY open up the multi-tasking interface instead of backing out of the app you are using.
Again, these are widespread issues that have been acknowledged by Nokia and they say they are working on them. I honestly don't expect the upcoming update to fix any of these issues as that update is already completed. That's not to say it won't be but I don't know.
Overall, it's the best phone I've ever hard. It's bold, unique, and amazing. There are just a few issues and almost all phones have at least a couple.