I have read through the entire Marked Scourge and in general I think we're making it out to be much more of a problem then it will be. Yeah you could build an entire character around abusing it and it could give you a good damage bonus with the burst 1 + action point combo.
But I don't think it's broken for two reasons. One the circumstance where you are going to be adjacent to much more then say... three enemies is super rare, at least in my experience. The other reason I think it's a good feat but not broken is because in order to even get close to "abusing it" like Covaithe said you need to be surrounded by level appropriate enemies. And I suppose that to me is also why it's a Defender thing. Only a Defender would be able to survive that.
So I guess my feeling is that it's fine as is because it just doesn't seem to me that it would come up often enough to be game breaking, especially when you have to focus so much on it and construct this multi adjacent enemy situation to really take advantage of it.
Like all of us its just my two cents but I cant help myself