No fault of the iMac...just that the thunderbolts are horrible in comparison.
I have a TB display at work, a 27" 2010 iMac at home and a 27" 2013 iMac at home. Personally I feel all three displays are of virtually equal high quality. Perhaps not what a photographer needs, but for my use they're great. The 2013 iMac does have a bit less glare, though my lighting is well controlled and it's essentially a non-issue.
Absence of USB 3.0 on the back of the TB display is perhaps a minor negative, but a quality USB 3.0 hub eliminates the problem (yes, no one wants to spend more money than necessary, but a good hub is only at most 4% of the cost of the Thunderbolt Display).