I think it has a lot of momentum, so it's unlikely another cryptocurrency will supercede it. The basic rules of Bitcoin are set in stone (e.g 21Million limit) yet it is still possible by community agreement to evolve it.
It's less anonymous than many people think, and ultimately governments will probably put some sort of 'tagging' system on it which at least stops it being too effective for use in certain crimes such as kidnappings/extortion. For the average user though - it will probably remain fairly private. It's probably a good thing that bitcoin is *somewhat* traceable, as in the end many governments will realize they are better off regulating it than attempting to ban it. (which would be as ineffective as banning file sharing)
Even if it never goes 'mainstream' - there are enough enthusiasts and important uses for Bitcoin that it looks like it will always have a niche. It is a form of 'programmable money', which is a very new and interesting thing. Not only humans will have bitcoin wallets - software agents could be sent out onto the net with control of their own income and expenditure.
It may not become super valuable as some of the more excitable people seem to think - but over the years I expect a gradual rise in price and a bright future.