The ACD is brighter and white looks whiter to me, if that makes sense. Overall the ACD is just more vivid. The iMac is a very nice panel, though, just not as nice as the ACD, IMO.
Also the LED is instantly at full brightness when turned on whereas the iMac takes about 10 minutes at least to warm up to maximum brightness. I didn't think it would be a big deal at first but it is nice to turn on the computer and be at max brightness without waiting for warm up.