It tests a wide range of web standards, some of which are very infrequently used. The list is on wiki if you're interested. Firefox doesn't pass because it doesn't support SVG fonts properly, though I'm not sure I've ever encountered a site which needs SVG fonts. It's still something which should have been fixed, though.
Sites only display properly in IE because any non-trivial site will use CSS hacks to work around IE's poor standards support (much less so in IE8, but you can scarcely code anything without having to hack for IE6/7.)However, despite the fact that FF et al score so much better than IE, I can't say i've ever had problems with a site not displaying correctly on just IE