Common faults are usb ports not working and on/off buttons (the one on the front) not working.
That seems to be 90% of the warranty reasons I have seen on forums.
Just make sure you buy one still in warranty as one quick phone call later, you get a replacement delivered next day and they collect the old one the day after.
Only downside for you is that recently since it is a discontinued model, NEC have being shipping 24" wmgx3 as replacements which although is very nice, it won't help you if it doesn't fit.
Although if that happens you could always sell it and buy another 20" one and pocket the change
Oh and as for Sid's comment on it being old by modern standards (it is only 3 years since they started making them), no other manufacturer has yet to bring out a screen of as good quality. IPS screens are still few and far between and most suffer from ghosting in games but NEC got it bang on right with theirs.
EDIT: One last comment. The glass glossy highly reflective screen on the NEC is not to some people's liking. I loved it and it makes the colours have so much more punch than other screens (even better than my new 24wmgx3) but not so good if you have a window or light source directly behind you.
