Just because something is possible with all classes doesn't mean some don't perform a bit better than others at it. I mean, really, some classes just have more of their top mob grinding skills at low levels than others do. Warlocks for example have Immolate, Life Tap, Corruption, Summon Imp, Shadow Bolt, and Curse of Agony by level 8 and by level 14 they add in Drain Life, which turns grinding down time into a virtual non-issue for them. Meanwhile Hunters have no pet prior to ten and used to have that damned dead zone eating up an extra autoshot. None of the classes are bad enough at low levels to make you think "Wow, this class sucks," before giving up, but you could easily be forgiven for thinking "Man, class X just doesn't seem to have a lot going for it compared to class Y." Granted, this is less true now than ever before thanks to all of the tweaks Blizzard has made since release, but I think it's a bit myopic to act like there isn't some meaningful differences sprinkled around at various points.
[Edit]
Sometimes people in WoW scare me. I have a few bank alts specialized in various types of items for the sake of organization. One of which is a female human level 10 warrior named Bankette. For whatever reason, she gets more guild invites than the rest of my characters combined. They always seem vaguely surprised when I decline saying it's just a bank alt.