I agree with other posters who say it's probably a bit too early to be making judgement calls like this. Unless you got an advance copy at GenCon and have been playing the game every day since then, I doubt you've got enough plays under your belt to really say for sure how the game is balanced.
Having said that, I can certainly see how the Overlord position would appear weakened in 2e compared to his position in 1e, based on reading the rulebook. As the OP mentioned, he has generally fewer monsters, new and potentially quest-based objectives (not simply "kill the heroes a lot,") etc. However, I'm hoping that this just means the OL needs to try a bit harder in 2e. After OL players have had a chance to ramp up, my hope is that the OL and hero teams will be roughly equal. (For what it's worth, reviews I've heard coming out of Realms - games where the OL was, by and large, played by an FFG staffer - seem to indicate that the game is well balanced, with the question of who will win hanging in the balance until late in the game. Barring a few reports where the OL or the heroes made a serious tactical error, of course.)
In 1e, the Overlord had a pretty easy learning curve. Heroes had a lot more counter-intuitive tactics to pick up on before they could play effectively, and they had a lot more random draws leading to sub-par character builds. In 2e, the randomness of hero development may not be entirely gone, but it does seem to be seriously reduced. Combine that with the fact that the Overlord appears to have a learning curve that requires some effort, and I can see how the game might seem weighted towards the heroes at first. ESPECIALLY on "First Blood," the self-proclaimed "tutorial quest." And on top of all that, the hero tactics are mostly unchanged from what I can tell. Some of the cheesier moves are discouraged, but the basics are still perfectly functional.
What that adds up to is an Overlord who needs to learn how to play a new game while the heroes (mostly) do the same thing they're used to from 1e. Given time to learn the new style, I suspect things will even out. Somewhat ironically, a group who never played 1e may find it easier to pick up 2e fresh and learn the ropes than a group who is accustomed to the 1e mechanics. I'm not shy about saying I think 1e's rules were sorely broken, so i don't necessarily see this as a bad thing, mind you.
Edit: I do agree that it seems like playing without the campaign is mostly pointless in this edition. Unless you don't have a lot of time and just want a quick one-shot Descent fix. Of course, the campaign mode was resoundingly what fans wanted out of 1e, so I don't blame FFG for going that route, either. I doubt our group will be much interested in playing non-campaign games anyway.
On the plus side, the rules for playing one-shot games at different power levels are fairly straightforward. If your group really feels that either the OL or the heroes are overpowered, it should be easy enough to handicap one team or the other by giving the "weaker" side a power boost using those guidelines. I wouldn't recommend doing this until you've played enough to say with confidence that one team or the other needs the handicap, though.