Oh, I'm going to be terrible at summing it up.
Basically, it's heavily influenced by Judith Butler (and unsubtle readings of Butler), as well as some others. Obviously the term itself originally had a lot to do with reclaiming 'queer', though it's now moved away from that.
As I understand it, the postive aspects are that gender and sex and sexuality are all things we can choose to act out in different ways. It's a theory that recognises and celebrates the richness of different experiences - so it's saying, there are lots of different ways to act, and they're all valid, and everything in the garden is lovely.
As I understand it, queer theory applied to stuff I know about tends to result in people assuming all transgressive behaviour is positive. You see, the idea is you're celebrating transgression as a powerful activity that challenges people's perceptions. Obviously that's good in itself, but it can IMO lead to people ignoring the realities of how transgressors are punished.
Lierre Keith's speech included lots of points that touched on this, but the one I understood best was her talking about the permeability of female bodies and the lack of boundaries female bodies have, in a patriarchial society. Basically, as we know, the patriarchy would love to say that all female bodies are permeable, in that they could all be penetrated and all women are interchangeable (sorry, I know this is really basic). But queer theory would try to say, maybe this permeability is a source of power ... maybe if we all transgress, we're changing society by acting against its norms.
The problem being, of course, that this doesn't necessarily work.
Anyway, I digress. As I understand it, the basic tenet of queer theory is that everything is to do with how you behave or identify yourself. There aren't absolutes, there are only spectums. In that sense it's postmodern. However, IMO applications of queer theory end up reinforcing the same limiting binaries as ever.
I am about to stop wittering on - honest - but the other issue I have with queer theory is that the category of 'queer' is expanded beyond what makes sense. I've had mates tell me I am a queer theorist because I am not a right-wing heterosexual defender of marriage.
IMO if we label most everyone as 'queer', it erases lesbians and gay men.