I kind of feel it's a mix of anything 10+ years old where stylistic elements or some aspects of gameplay have gone out of fashion. For example the Skeuomorphism of the X360 dash, the heavy use of gradients and pointless graphical effects and abstract sound on PS2 startup, right back to pixel art, these are somewhat outdated effects of their time. The sounds for the XB1 startup are now laughable, the GC startup sequence is oddly cutesy but the moire and rainbow sheen of the rotating cube there is just not in place today.
I expanded on that more than I expected but as a designer I'm probably going to think that way. Respective game aesthetics haven't really changed that much, a "photoreal" game is still going to be as "photoreal" as it could be on the PS1 as it is playing Forza 6 on XBO, just better/not as good. A Disney film is still Realist animation whether it was drawn on cell and watercolour tablets on drawn on a computer.
The technology gets better but styles are still consistent. So for a game to be retro to me it has to be at least one of both, if it's both it's definitely retro. We might simply replicate these aesthetics in the future but they are still "retro". In essence the term has two different meanings, but I wouldn't call being into games like Towerfall, the original BIT TRIP Runner or Resogun "retro gaming".
An interesting aside on that is the Wii U interface design was outdated aesthetically the day it launched because while Nintendo were originally emulating Apple they didn't have an original aesthetic to go on so they just carried on with Wii's - one very small of many factors why that console failed, because the brand and the look was off. That's how video games tie in fashion and design trends and can have a pretty strong impact on how they succeed.
For example, the western and eastern designs for the NES are so different because it was originally designed to look like a VCR, along with the name "Nintendo Entertainment System" because Nintendo wanted to distance themselves from the toy-like and furniture-esque aesthetics of the home computer generation. Nintendo were actually using the same strategy as Sony did with the PS1 except 10 years earlier. It's the same but bad gaffe MS made with the original XBox which looked ridiculous and now makes no sense in the living room and very much a retro-looking machine. You don't really get any timeless console designs which is why retro gamers in general are so tied to the physical aspects of gaming like hardware and shelves of disks etc. because it's an important part of the very definition of "retro". With the hobby floating on the economics of the second hand market of old stock that's also an intrinsic element, so if you didn't have to go somehow out of your way to play a game like set up an emulator or hunt down a physical copy, I don't quite consider that somebody "retro gaming", as it's part of the experience. If that weren't true we wouldn't see replica consoles or microconsoles like NES Mini, it's a market created almost entirely by nostalgia (and some might say quality of gameplay
) which is largely due to the kinaesthetic experience of playing these older games adding to their quality.