From my personal experience as an Indian with an Indian passport and lots of friends who don't have American or European passports, visas, even for holidays, are the biggest travelling pain. Suffice to say, I travel less because I hate applying for visas, not least because finding the correct information is really hard. If you took some of the pain out of that problem, you will get a lot of users.
I know it's not as sexy as making a social network for travellers who can check in to cool places around the world.
But, it's a real problem and will really help people.