Just got my key from GMG as well. I'm pretty hype. Here's hoping the institute won't just be evul technonazis and the brotherhood won't just be goody paladins of justice and compassion.
On the brotherhood front, the launch trailer shows the player fighting brotherhood members, so they may be more in line with stuff from the older fallouts/NV.
They seemed perfectly nice in Fallout 2 :/ They distribute useful technology to help people, they just try to archive it first. They want to eventually rebuild the world's high-tech infrastructure. In the meantime they try to keep advanced military technology out of the hands of bandits.
(From what I understand, in Fallout 1 they give you a quest they don't expect you to complete. But when you do, they're happy and make you a member, and the quest was actually very important)
They recruit from locals, too. Mostly in Fallout Tactics, but it's canon that they do it everywhere. Anybody can join the Brotherhood if they're useful enough to research or protect.
The NCR-Brotherhood war bothered me a lot, since I didn't see that coming at all from Fallout 2. But I guess good organizations can get corrupted, or (as the non-canon Van Buren indicated) have a radical leader come to power. As sad as it was, the conflict made New Vegas more interesting... If the Brotherhood had become an arm of the NCR like the Rangers, that would have been boring. Instead we have a bitter blood-war between two good factions.
Also I notice that people seem to misremember the NV ending slides, thinking that the Brotherhood always ends up raiding civilians for technology (something that just doesn't happen in Fallout 1 or 2, and only the Outcasts do it in 3). If the NCR wins the dam, and there's a truce, the Brotherhood just helps the NCR patrol the roads.
If you establish an independent New Vegas, they do reclaim or steal technology from travellers. Which really bugs me, because I don't think that really matches their lore at all. I suppose it could be because House tried to kill them without any warning or negotiation. So they might feel like the Independent Vegas isn't trustworthy enough for high technology.
So, what to expect from the Boston Brotherhood... 6 years after New Vegas, 10 years after Fallout 3. The East Coast Brotherhood is almost naively kind and generous, so why would they be enemies in the teaser? The Brotherhood Outcasts. Chased out of the Capital Wasteland, they supposedly were headed back to the West Coast. But it'd make more sense for them to head up to MIT. They'd make great villains, with their open disdain for non-members and their hoarded technology... And basically being a corrupted Brotherhood, lacking empathy.
Their numbers were low, but maybe refugees from California have arrived. That'd add some moral depth, since the NCR was literally hunting down and killing all Brotherhood members they could find. The refugees would have a reason to be xenophobic (something the Outcasts kinda lacked, they were just "Hurr we're the best so we should have everything").
Hopefully it's possible to negotiate a peace, at least in the late game. The Capital Wasteland Brotherhood would want to help with that.