It's not that the story didn't end. It did. It could have been wrapped up nicely. But Bendis doesn't like concluding a stories and moving on. So he had to introduce a new plot threat at the end that was completely open ended so he could leave it dangling. I honestly think he has no idea what he's doing most of the time, he just thinks, "What can I include that's vaguely mysterious enough that I can make people wonder whats going on, and open-ended enough that I don't need to decide where it's going yet?" Then he tries to tie it all together later, telling people it's been planned out since the first issue, but he usually does such a sloppy job that its obvious it wasn't planned out from the beginning (I'm looking at you Secret Invasion!)
I feel like this has mostly worked for USM (although its been frustrating at times b/c certain things were never addressed) but it hasn't rally been a good thing in any of his other books - except for maybe seeing the ongoing character stories of Luke Cage and Jessica Jones.
New Avengers is another perfect example of this. Part of that was editorial mandate, I'm sure, to tie in to whatever current crossovers were going on, but it was still a huge mess.