I posted a thread about this back in Aug 07 and I'm very glad to see that the topic is getting a lot more attention.
I will cover a few points that people should consider.
1. I believe that certain specific extensions have a very good shot of becoming popular. These would mostly be extensions that would cover an entire industry, such as .doctor, .realestate, .bank and ones in a similar vein. It's definitely not guaranteed that this will happen, but I think that one of these extensions in the right hands, with the right marketing and sales could create an industry specific extension that is adopted by the entire industry. If you look at the way that the Internet has been progressing, we have created more and more extensions for grouping into specific categories, whether geographical or other. Some ccTLDs are already more popular than the .com in that specific country (.de, .co.uk, etc...) So, I don't think it's impossible for this to happen for a specific industry as well.
2. I think a lot of companies will be applying for their own extensions. If you think about it, even with the application costs and the costs to maintain your own registry, with a few key registrar partners and a landrush auction system much like .asia, you could bank some major profits within the first 3 months. I think we will experience a lot of "pump and dump" operations who are just in it for the initial public offering. But then again, with some of the extensions we have now, you really couldn't say much more about the registry (my example would cause a fight, so I'll refrain from saying it, but theres only one extension that could really do that, so I'm not being too subtle).
3. Aside from the rare cases (if it even happens) where an extension is adopted by an entire industry, I think that generally speaking the influx of new extensions will do nothing but help increase the value of .com and lower the value of other gTLDs.
4. I think these message forums will be inundated with appraisal threads and new extension discussion threads and it saddens me.