The same as with LCD TV's. LED TV's are still LCD TV's, but they have LED backlighting instead of the fluorescent backlighting of LCD's.
1) Poor viewing angle on the sides.
2) Not the best with fast motion sports.
Screen uniformity ... cloudy blacks ... and edges that are brighter than the center of the screen.
Glare ... on the higher end Samsung and LG models ... worse than anything else on the market.
I currently have a Samsung 8500 purchased a year before the 3D revolution happened. I can honestly say I have very few complaints. I don't have any of the issues stated above. Viewing angle is amazing, contrast/brightness is so good that when I calibrated it I had to bump it down, way below factory settings. And as far as the motion goes. For example: NFL games are super clear, even with the 240hz turned off. Take your time finding a great display, and weigh all of your options. Plasma or LED, there are some amazing displays out there.