I don't think the plot went from Great in ME1 to abysmal in ME3 like a lot are saying. The ending I think people are overreacting to like they do with a lot of games. I mean I have issues with the ending, but I generally have issues with the ending in most things. I don't think the plot was that drastically different in quality than the first 3.
I like this game a lot, but not as much as the other two in most departments. There are some improvesments that I thought were neat. But there were a lot of niggling things that combined to bother me a fair amount. The level design bothered me more too, because it didn't seemed improved, which was my major issue I had with ME2.