I don't think that new extensions are going to affect .com much. Dot com has already fared well against many alternative extensions and stayed strong.
The new extensions may well hurt other gTLDs, especially ones that are not well established like .info and .mobi. This would simply be because there would be more competition and more alternative extensions for people to choose from.
I can't see much impact on .in from the new extensions. Dot IN serves a purpose - to show that a website is Indian and caters to the Indian market - and that is not going to change with new gTLDs.
One thing that may impact .in is if the larger cities in India get their own local extensions. Given the cost and technical complexity involved, I doubt that many will try this in the near future.
There could also be IDN extensions that impact .in - say a Hindi extension. That may have a bit of a negative impact.
For domainers, I think that there will be a lot more new opportunities - but also a lot more people will lose money buying into new extensions that don't work out. I'm a pretty conservative investor, so I'll be sticking to the tried and true - which includes ccTLDs like .in.
My 2 cents! It is pretty hard to predict the future on something as major as this.
Well, I am not sure one can register their domain with their own TLD. But, it is not going to hurt the sale of .in as .in and .co.in are something popularly used TLDs and not all would uniqueness in TLD's