Republic vs Empire: Which tends to be more active and more willing to group? I've noticed that as a republic player, finding open world groups for Heroics tends to be a pain and most of the time you simply get a "gain more levels so you can solo it" as a response.
I've played both Rep and Imp on the Harbinger, and it's been reasonably easy to group for Heroics on both sides. Of course, there are times when I can't find a group, but I have been able to do the vast majority of Heroics on all of my toons (and I have no guild, and none of my friends play).
As far as the "gain more levels so you can solo it" responses, ignore them. Such remarks are made by trolls who are just trying to stroke their own egos at your expense. Be careful what you wish for: a more populated server will also contain a higher number of fools.
You are correct that some groups have a low tolerance for newbs. But as a previous poster said, if you tell your group up front that you're new to tanking, and that you're willing to learn, most people will help you out and be much more forgiving.
If it makes you feel any better, tanks aren't the only ones who get mercilessly chastised. Healers can get just as much harassment.
My main is a Guardian, and I love it. Yes, area aggro is difficult to deal with. But single-enemy aggro is a breeze, particularly with the hybrid Defense/Vigilence spec I use. And as a previous poster said, when faced with area aggro issues, I enjoy the challenge.