Upgrades and choosing the right Mac are an interesting topic to me.
Despite the slow overall uptake for Thunderbolt, it presents some unique opportunities for those of us who buy all-in-one machines. We can actually plan to have better, cheaper, larger high-speed storage in the future without having to perform surgery on the box itself. $1000 for a 1TB SSD is a lot to spend. Not only do you have Apple's usual markup, but SSDs of that size are still new and a bit of a novelty. The value just isn't there, even if the money isn't an issue.
Likewise, I'm always a little surprised how many people (at least here on MacRumors) choose both the graphics and processor upgrades.
$200 for the i7 is a ridiculous amount of money for what amounts to an indistinguishable speed increase—with the exception of a few, narrowly-defined, highly-parallel tasks. If you do certain types of processor-constrained 3D rendering, or a lot of video encoding (h.264 in particular; most other codecs benefit less), then the i7 upgrade is a good idea. Almost all other tasks (including much of the CreativeSuite besides AfterEffects rendering, games, all basic tasks like browsing, etc.) are speed-constrained and no number of extra cores will solve that. It used to be the i7 upgrade also bought you a reasonable increase in speed over the baseline option—so if you had the money, you could at least say you were getting a GHz premium, even if hyperthreading didn't benefit how you use your computer. But considering the difference in both base speed and TurboBoost between the top-tier i5 and i7 is just 100MHz, it's not even worth considering for speed alone. The only reason to upgrade to the i7 is because you need the additional, virtual cores.
This doesn't mean we should all still be running single-core processors. OS X takes good advantage of a multi-core system, but those benefits drop off sharply after 4 cores. Even the gains in going from 2 cores to 4 isn't as big as you would think—the other advances in modern processors (larger caches, on-die MMU, etc.) have done more to improve performance since the Core2Duo days than extra cores.
The graphics update is similarly over-sold, especially in the latest iMacs. If you go by specs alone, the 780M seems to be a significant upgrade from the 775M in the top-tier, $1999 27" iMac. Yet, benchmarks show there's only a moderate difference between the two. What made the 680MX such a worthwhile upgrade in the 2012 iMacs was the 2GB of VRAM, allowing the GPU to keep more textures "close by" without having to page it out to system RAM and destroy performance (that Apple still sold Macs in 2013 with 512MB worth of VRAM is ridiculous. That's barely enough to do basic window compositing for a 2650x1440 resolution screen without paging). Now that the top GPU options have 2GB or 4GB, the advantage is smaller, and you're more likely to run into processor speed constraints than GPU constraints. That being said, texture-rich, AAA-level games will benefit from the 780M's ability to throw textures around faster. If you're a hardcore gamer, use Windows in Bootcamp to play them, and can settle for nothing less than full-quality at native resolution, then it's a worthwhile upgrade. If you're not, I have a hard time seeing the value, even for future proofing.
Ps. I don't play games.