Limbo.
I love games which can be interpreteted in different ways, Braid being a prime example, but there has to be at least SOME indication and direction to what it means.
Heck, the only reason you knew the girl was your sister was because it was in the game description and then mentioned literally nowhere else.
Now if the end had been sort of a plot twist, and you realise your actually at the start of the game again, and the idea is that Limbo is you just going around in circles and enduring multiple painful deaths as part of your punishment IN Limbo, that would have been epic.
But sadly, it was a open ended thing which made no sense and had no direction for speculation.
TerribleAssassin said:
Assassin's Creed 2's.
No real ending just a crappy cliffhanger with awful DLC to make up for it.
Did you do the glyphs beforehand? If not, then the ending won't make nowhere near as much sense. I personally loved the end for AC