The superdrive was definitely an unreliable part, for sure, and for all optical drive related problems simply not having that internal optical drive meant less problems, but in terms of reliability with the rest of the computer it had a very little effect. The optical drives used were incredibly thin and took up practically no space as it stood. It was also not exactly a heat generating component compared to the gamer-laptop-PC style graphics board, or the 3.5" hard drive.
Plus they made it much thinner than the thickness of that drive, more than canceling out whatever effect simply having something in that area would take up. The 27" iMac doesn't appear to do much of anything to fix the heat issues; they have a similarly heat-generating video board, a similarly heat-generating 3.5" Hard drive, a similarly heat generating desktop CPU and desktop north and south bridge chipset chips. These generate heat. They removed the optical drive and replaced it with more thinness than it took up to begin with, so no, I'm not confident that they will have fixed the thermal issues with the 27" iMac. However, with the 21.5" iMac switching to 2.5" hard drives (5400RPM drives at that) thermals will go way down, similarly those machines don't appear to use the gamer-laptop PC boards that the 27" has used and still uses; so, laptop drive, laptop graphics, presumably laptop north and south bridge, the only real heat-generating thing to worry about is the CPU, which I think is still a desktop variant and not a mobile variant. Still though, that ought to not be a problem.
Screen issues I'll wait to form an opinion until they come out and not only I can see them, but others can review them and, most importantly, until the service bulletins on GSX indicate no known issues. Then I'll celebrate the improved screen.
It depends on what you do. If you're a gamer, then yes, the 256MB of VRAM will enable you to have run a greater number of games by the time it's naturally time to replace the computer. However, if you're a light user, then the ability to stay current with the OS is the single best way to future-proof it and five of the last six OS upgrades have doubled the minimum RAM requirement. The only reason why this one didn't was because it wasn't that large of an update in terms of features that required more RAM; unusual for Apple. They only kicked out ATI Radeon X1xxx cards, the NVIDIA GeForce 7300 GT, and the Intel GMA 950/X3100 cards because there were no 64-bit OS X drivers for them and there wasn't ever going to be, and given that Mountain Lion abandoned the older 32-bit kernel in favor of only having the 64-bit kernel (Lion and Snow Leopard both had both kernels where Leopard only had the 32-bit and where Mountain Lion only has the 64-bit), so 64-bit drivers were needed. Otherwise, that's the only reason why video cards were an issue this time. Usually with OS upgrades, video cards have no bearing on minimum system requirements. Ultimately, if one is buying a 21.5" iMac, since they can't upgrade the RAM later and given that the RAM WILL correlate in some way or another to the maximum lifespan of the computer, it should be maxed at the time of purchase. Similarly, I would ALSO get the best video card offered as well as the best storage option (GeForce GT 650M and 1TB Fusion respectively) as those also can't be changed later. Though, history shows that RAM will affect the ability to run a new OS more than any other attribute save for generation (which is irrelevant to this discussion anyway).