OK, I looked this up, Defence of The Ancients abbreviation. It says on Wikipedia that Dota-Allstars.com has 1.5m registered members and gets 1m unique visitors per month so there is alot of interest in this online. The question is will that interest spill over into the .pro domain?
I can see 3 main advantages of owning Dota.pro over other gTLD's. 1) Cheaper, 2) Less likely to get pulled up on TM issues because owners wouldn't consider the .pro much of a threat, and 3) Gamers in general likes the whole Pro branding thing. I bought Game.pro on this basis.
The thing to realise about .pro is it's a niche domain extension. People on domain forums know about it, but those people are in a small minority of the total population. Out of the people on this forum, most people stick to .com, that cuts down your potential market, then out of the remaining forum posters, most prefer alternative extensions other than .pro, then of the people who would consider .pro from a branding angle, many aren't eligible to register it.
OK, so you've narrowed things down now pretty small, how many people in your remaining sample play Dota and have an interest in developing a Dota website? The point I'm making is if you are going to pay the .pro game you have to keep it mainstream. If you can offer people a keyword that thousands would covet in .com but keep it brandable and a gTLD in .pro, you have a very small chance of the keyword demand flowing down to .pro. I have 300 mainstream .pro keywords, often words that appeal to a professional audience or keywords that sound the bees knees paired with .pro. I get fairly regular offers, very occasionally I get an offer worth accepting, but its a modest one in the context of .com.
You register some pretty niche keywords in .pro but have you ever sold one? In terms of valuation, what you can or have sold a .pro for should be your best indication of value. For something like Dota.pro, it's got to be reg fee reseller. To sell Dota.pro to an end user, you'd need 1,000 .pros exactly like it costing you $20 a year, and then you might sell one of them for $889 which is what Joomla.pro sold for in Sep 08. So $20,000 out, $889 in. Can you see how you will never make it pay? The situation with .pro is if you have the best keywords, you can cover your costs on average, if you are lucky you can make it pay just but that's about it.
Things might pick up in .pro if the registry ever completely liberalise the extension, and I think they will or they will go bust, but for now if you feel like registering something like Dota.pro with a view to resale, you might as well mail RegistryPro a $20 bill in an envelope and save yourself the trouble of registering the domain and letting it drop in 2011. If you are a Dota website developer, $20 is money well spent because Dota.pro is very brandable and if you have content worth reading people won't hold the .pro extension against you.