There's a nice example of button labeling on Twitter that shows how having a bit of fun with language can actually increase usability.
When you try to block someone on Twitter, instead of a simple Yes/No confirmation screen, the user is presented with this:
Typically, users have been trained to quickly click through confirmation messages without reading them. (Case in point, this is my second post on this subject as I accidentally deleted the original). By changing up the language, the user is given pause to consider their actions. The use of fun language to convey the message changes a functional step into a pleasant experience.
The subtle use of all caps in the first button also does a good job of conveying the permanence of the action while the sentence case on the second button reflects the mood behind it.