The engine is very old and the tech no longer fancy, but I still absolutely adore the way WoW looks. Older areas and the older races (which need updating as they look almost out of place) are obvious blemishes but the newer designs from post-Vanilla especially moving forward to WOTLK and Cataclysm show a really tremendous aptitude for making a massive amount out of minor tweaks, heavy art design and general texture blending madness.
The whole aesthetic is just great, to me, and I find it fits amazingly well with the type of art they use and the huge, sprawling scope and pace of the game. As someone who is a little blah on how a lot of games are progressing in visual style, it's a nice mainstay.
Of course it'll be different for every person but for me all I really care about is the art direction, the image quality/resolution and technology to be of a competent enough level to do a full realization of the game world they're going for... and to mostly avoid normal mapping. I am really, really sick of normal mapping.