There are at least two more expansions slated, so there will continue to be new content until at least 2014-5. That's a five-year business plan that was leaked though, so that's hardly expected to be the final expansion. To answer your question more directly, as long as it's profitable to continue to operate servers and develop/release new content. Considering Blizzard Activision makes more money than Jesus off WoW, I'd expect it to be around for a very long time.
Great time, just wait for the WotLK battle chest to get released so you're not dry-humping yourself out of $40-60. Cataclysm revamped all 1-60 content and it's the beginning of a new expansion cycle, so there are lots of new and rejoining players making new characters. The new content is pretty awesome and it is very noob-friendly.
By this question I'm assuming you mean the 80-85 content. Yeah, it's really good.
Now that Taurens can be Paladins and the entire lore of the game has been spat on by its own developers... It wont be long until the real players of WoW quit and leave behind all the idiot 12 year olds who run around spamming 'holy cow lololol11'
Oh, quit crying. The game doesn't exist in a vacuum where things never change and lore is static. One of the central themes in the content releases is that times change and races and factions adapt. IMO, the lore behind tauren priests and paladins is some of the best-fitting, most appropriate and welcome Blizzard's lore team has thought up so far.