The Gnomes basically resemble little people: very small stature, large heads. They’re cute, but not conventionally beautiful (like the Humans and Night-/Blood Elves). [7. Annie chose a Gnome for her main character, Beatrixkiddo, primarily because the female Gnomes didn’t look like the Humans and Night Elves.]
The male Gnome dance comes from Bloodhound Gang’s video for “Bad Touch.”
The female dance’s source isn’t as clear, though WoWWiki attributes it to salsa.
High-pitched, nasal SAE. Basically, they sound like nerds, which fits their lore.
They’re geeks. The Gnomes’ most notable feature, which infuses their architecture, narratives, racial abilities, etc., is their interest in engineering. Gnome areas are full of mechanical gizmos; even their mounts are large gas-powered ostriches. Their technological expertise is portrayed with a lot of ambivalence, though. On one hand, they’re described as being brilliant engineers, having built most of the Alliance’s technological wonders, like the Deeprun Tram. But on the other hand, it’s responsible for a lot of mishaps: a lot of crashed Gnome airplanes dot the landscape, and the Gnomes were actually forced to evacuate their home city, Gnomeregan, because their overzealous excavation awakened some evil Troggs. Now the Gnomes are basically wards of the Dwarves, occupying a small section of Ironforge called Tinker Town. So their love of technology is presented as cute and humorous at best, and as foolish at worst.
The ambivalence about engineering is an interesting feature of the Gnomes’ design because it resonates with the ways technology is presented in high fantasy. To the extent that it’s presented negatively, it fits with the Tolkienistic representation of technology: that it’s destructive, especially to nature. But in Tolkien, the industrialists are all unequivocally the bad guys, and in WoW, Gnomes are on the good guys’ side. So tech isn’t all bad here.
They’re played for laughs. There are a lot of references in the quests and NPCs’ utterances that make light of the Gnomes’ size and general goofiness. I’ve noticed that players of other-raced avatars tend to refer to Gnome avatars diminutively; it was a favorite joke of Dave’s in idle moments to turn in to bear form with his druid and sit on Annie, which totally obscured her. This is all playing into pop-culture representations of little people (often referred to as midgets) – n.b. Mini-Me in Austin Powers, Jackass’s Wee Man, the Munchkins of good old Wizard of Oz.