I think there are still improvements that could be made to TVs that would probably tempt me to upgrade a little earlier.
Remote controls still suck. I connect a Raspberry Pi to my TV and it gets input from the TV's remote via the HDMI cable and CDC protocol. The remote is not very responsive and far from ideal for controlling XBMC, or even smart TV apps for that matter.
Better sound would be nice too. I have a sound bar and sub, but most of the time I'd be happy with TV sound if it was as good as my old CRTs. It's hard to see how it could be done on flat screens, but I'd happily accept an extra 10-20cm depth for really good sound. Maybe we could have a universal standard for wireless sound too, so I don't need to wire in my sound bar and sub.
Bring back plasma. Panasonic were the last people to make good plasma TVs, but gave up because the cost of upgrading to 4k was just too high. The colour on plasma still can't be beat. Maybe if someone can create an LED TV that is as good I'd be interested, but for the moment I'm worried that when my current plasma dies I'll be forced to downgrade.
Add automatic muting during ad breaks. I rarely see them, but I want it anyway.