I agree that there's something vaguely unsatisfying aboutthe graveyard scene. Maybe just because there's so much going on that it's not fully digestible, but not in a fun "cocktail party" sort of way a la Pirates. One definite point in Pirates' favor is that you get a solid amount of ride in before you get to the Pirates, and then once the Pirates finally show up you're with them a good long while too. You have plenty of time to digest their antics, and while there are a number of figures they are spread out enough and you have enough time to be with each scene that it doesn't feel like overload. By contrast, the graveyard can feel like their mentality was "well, we have all these ideas, and only this much room, but we'll just squeeze it all in there and make it work." So you end up with one room with tons of animatronics, all with their own business they're performing, and while it's kind of cool, it's too much to process in too short a time.
I think the graveyard is necessary in that the entire ride is spent teasing the idea of ghosts, and wondering what their intentions are. While the ballroom and the attic start revealing ghosts, they don't reveal *intentions* per se and they could still be a threat to us. The graveyard, then, not only gives us a potpourri of ghosts that satisfies in quantity so that the rider doesn't feel cheated, but also resolving the central tension decisively. It could be argued whether or not that resolve is necessary (as there are plenty of scary films/mazes/etc that don't offer any resolution whatsoever), but it does give the ride a more definitive conclusion.
I agree that the time in the doom buggy is too short at DL. The extra scenes they put into Florida and Tokyo aren't all that monumental on their own, but they DO make it feel like the ride portion has a little more room to breathe, and that translates to a superior ridethrough experience, IMO. The tradeoff for the longer ride time, though, is a less impressive stretching room and the lack of the portrait hallway walkthrough (though you travel through it via doombuggy). If only Disney had built a version of the mansion with an elevator stretch room, a portrait walkthrough, AND the extended ridethrough scenes-that version of the Mansion would be unbeatable!