You also have a way better internal chip situation.
ST60 was artificially capped out of the chip department both as a cost-saving effort and because UT50 and ST50 most likely cannibalized last years upper sales market by being on-par with them in lots of things; actually surpassing them in others.
For starters, Plasma as a product is a glass sandwich, the most layers you have to it the darker the screen will be (you can account for their increase through the gradation levels), each layer usually means you have an extra magnesium oxide layer, which is used to emit extra light, that's why the plasma design is really not meant for energy class wins, there's pretty much a war going on inside them, with multiple agents and repetitions. They also end up being a very intrinsic engineering wonder, but I digress.
Anywho, despite those compensation shenanigans, VT60 is darker than ST60, thus ST60 will look way better at half life (100.000 hours of use past, if it gets there) than a VT60. That much is a given (and the filters are applied as a post-treatment, it's like nail/car varnish; not part of the layer itself, I could turn my X50 into a antigloss panel myself with 3M solutions and the like, albeit less efficiently than the solutions researched and employed by Panasonic), last year though, VT50 was a tad dim only hitting about 100 cd/m^2 and ST50/UT50 was precisely where it should be (~130 cd/m^2). This years ST60 goes as high as 165 cd/m^2, sacrificing nothing; it's not that you will use a TV with the highest settings, but seeing light decreases with the lifespan, half life is the phenomena when the original 120 cd/m^2 get reduced to 60 cd/m^2 in that scenario, the ST60 will still be able to give an image as good as a calibrated VT50. (usually in the ~80cd/m^2 range) that's huge considering these TV's are rated 100.000 hours till half-life.
Enough drooling over the achievement, but... this years ST60 is such a big jump it humiliates the VT50 (that's not always the case with next years middle range against last years top range); you'd objectively have no reason to buy a more expensive set, if the hardware surrounding the panel was like last years; and that was my point in doing the detour above. The improvements in peak light were meant for the more complex sandwich this year of course (VT and ZT60) but in the end they benefitted the ST60 again; peak light being more palpable than more gradation levels.
So, they cut on the chips. Last years chips were custom dual core Cortex-A9 @ 1 GHz for UT/ST50 and 512 MB of RAM, GT50 and VT50 had the same SoC overclocked to 1.4 GHz and 1 GB of RAM. Same chip heritage, only slightly capped; response time was unchanged and could in-fact be better due to less post processing. This year... you have a Mediatek 5590 that really can't keep up with last years UT/ST50 in response (it still offers the same functionality, though, just clearly slower at it's trade); GT60 though, has the same chips VT60 has. As previously said though ST60 humiliates a VT50 in picture quality, and the VT50 humiliated everything else that year too.
GT60 is a ST60 *plus* with VT60 chips that negates every ST60 shortcoming. It's worth it considering the gap there was on the chip department this year; last years GT50's weren't all that interesting by comparison.