So, as you might already know, those who can afford a gTLD are going to be able to get it soon.
I can already find people who are opening up for preorders on gTLDs like
.deal, .movie, .food, .free, .music, .golf, .news
.berlin, .london, .miami, .dallas
.family, .arab, .gal
So, will this slow down the .COM ? in my oppinion, nothing can slow down the .COM, i always advise people to always get the .COM even if they want there main website to be .net.
The reason is, when people want to visit you directly, they will either google your business name, or enter it followed by a .COM, if you get the .NET and no .COM to forward to it you will probably be loosing visitors to the competitor who has the .COM.
Othe gTLDs did not result in the revolution everyone expected, Always a Gold Rush, then 1 year after, people would not even bother renew the domain, if it is not a .COM, it is hard to make it work for you.
I do think the ICANN’s idea of approving any gTLD for anyone is a good idea, for example you will probably be able to download your drivers from www.drivers.hp but i don’t think HP would ever drop the HP.COM website or let’s say depreciate it.
Even if some gTLDs do pick up, i don’t expect anything revolutionary, nothing capable of stopping the .COM growth, but i do think they will have there place, some place at least.