I can definetly see why some people dislike it, for two whole games (and most of the third) the focus has been on Shepard and his/her crew, and on a larger scale, humanity. In the end the focus shiftes to a much larger scale. The Reaper on Rannoch says it first, that the extinction cycle is means to maintain order in the galaxy, then the Catalyst in the end says much the same. Shepard and humanity builiding the Crucible and docking it with the Citadel proves that the cycle is no longer viable solution, and as the one who made the way of breaking the cycle possible, Shepard is the one the Catalyst entrust the future of the galaxy. Perhaps more than just the galaxy.
At that point it's no longer about Shepard, or the crew or even about humanity or the reapers, it's about the fate of the galaxy. And that is why the undetailed, open ending.
Or that's how i see it. The underlying reasons for the extinction cycle should've been elaborated more, but in the end i feel this ending works. It makes you think, and it's about much more than it initially seems.
And the best line of the ending? "Tell me another story about the Shepard." That really says it all
