I don't understand how some people claim that Standard Definition "hurts" their eyes. Yes, sometimes you get used to the HD levels of graphics, but some older games look just fine, and in some cases they look beautiful! (Granted, art style definitely makes a huge difference).
Recently I played Dirge Of Cerberus, a late PS2 game running at 480i using Component cables, and the cut scenes rendered with the IN GAME engine look GORGEOUS, the character design really shines and are all well animated, and this is a game that clearly went for some form of realism. 2D games age very well, take The Legendary Starfy... It holds up very nicely because the art style is so pretty.
While I agree that some games have aged horribly (Final Fantasy VII, Turok Evolution, Wave Race 64, etc), they're still playable in my opinion.
Maybe its just me, since I've been playing mostly PS1/PS2/PSP games for a while now, and I'm still amazed at how pretty some of them still look. 480i and 480p are more than acceptable for me, and I play on a HDTV.
Anyway, I can play older 3D games easily, but some of them (Deep Labyrinth on the 3DS) are just poorly animated and give me headaches. 2D wise I could play down to NES probably, I just don't like 2D games very much, much less the current gen indie shit like Shovel Knight. Give me any Contra, Castlevania, or Metal Slug and I'll play it for hours.