"There are only two hard things in Computer Science: cache invalidation and naming things" - Phil Karlton
For, http://www.mobtropolis.com, it came out of the concept of collectively finding interesting things to do that were out of the norm. If they were doing it all at the same time and place, it would be likened to a mob. I figured this sort of thing would only happen in more urban areas with denser concentration of people. Hence after a short brainstorming session..."Mobs in the City" became "Mobtropolis".
I hesitated for a while since it's a 4 syllable word, and people might not remember it. But since I couldn't find anything better, I just went with it.
After a while, a name just grows on you and others, as long as you're consistent about it. It also helps you when you're pitching it if you don't feel stupid saying it. I would imagine del.icio.us wasn't exactly easy to associate with bookmarking, but as long as people consistently associate a name with a clear idea of what your product is, I think it'll be ok.