We looked at act length in Diablo II and came to some of the same conclusions, "They ran out of time". But when considering it further the intent of the act structure actually got shorter as you went, intentionally rocketing you toward the end of the game. It's one of those things that makes game design hard to relate. It's... it's sort of like a movie score. If done right emotional music can make a scene feel 'better' or raise whatever emotion is being portrayed, but if you start scrutinizing the individual components, or just lose the immersion, it can come off as cheesy.
In any case I think we're estimating a longer game than Diablo II right now, regardless of actual comparable act size.
i knew i wasn't crazy, now all can see and know that if they have A2 done completely then they have more than 50% of the normal mode complete and since all difficulties have the same places and such makes the game a good amount complete. but of course its not coming out until AT LEAST summer 2011