I'm going to be awkward... I usually am... I think solid/good graphics is massively important.
Talking of MMOG's - EQ1 smacked UO into the ground on sales. Why? For years people said it was the player killing in UO, yet now PvP is THE thing in MMOG's. I think it's actually graphics that was the deciding factor. Okay views were different, and a variety of other things. Mechanics wise and game play I think UO had the edge on EQ. But because of the first person view and 3d graphics, I really think that's part of why EQ at the time dominated. UO looked old, at the time EQ looked new.
I know marketting budget has a massive effect on how well your game (or any product) does. I think though that "old" look is what meant a little know game called Conquest frontier wars by fever pitch didn't do well. It looked old. Sure it was 2d when everything was going 3d, but even then the "lines" of the units just looked dated. Sins of a Solar empire is the same game with capital ships tagg'ed on and some of the RTS resource gathering stripped out. As much as I like Sins, I think Conquest as a game has better mechanics and in some area's still has the edge game play. So why am I playing sins? Conquest looks butt ugly. If someone did a Conquest mod that played the same on the Sin engine, I'd honestly be playing that mod rather than Sins.
I would have told you a few years ago, it's all about game play. I was saying even recently "if it was just wire frame graphics, I'd play it to death if if had a good gameplay". In reality what I actually play, is all "pretty" grahics. As much as I said that, I'd be playing games that didn't make my eyes bleed.
Just as a final comment - just to clarify what I mean about good graphics. It isn't crysis, or what ever game requires the most hardcore graphics card. Zelda and WoW aren't top end graphics, but have an internal consistant art work.