Gnomes were added to Warcraft solely because of a hasty, not well thought out decision concerning Goblins and faction loyalty. They're used as comic relief but otherwise Blizzard acknowledges their fuckup and practically doesn't treat them as canon with their irrelevance from anything major up to cinematics. I mainly find Gnomes offensive because Warcraft as a franchise is founded on badassery and rule of cool, something Gnomes are devoid of. World of Warcraft is founded on the same themes, but also obviously with the MMO aspect of creating your own, unique and awesome character that progressively gets stronger.
It just seems to be undermining everything when you play as a Gnome compared to the likes of Orcs, Tauren, Night Elves, or even Humans.