Sentinel Prime waited so long because his ship crashed and he got deactivated. He was supposed to meet up with Megatron, but he crashed, got sent into stasis, and ergo had to wait until Prime reactivated him. After that, he waited until he had a chance to get away with the pillars. Also, the Autobots getting exiled was pretty stupid, but the reasoning was clear: humanity did not think they could beat the decepticons, and wanted the Autobots gone anyway. However, that was communicated poorly and came off as rushed. But at least you could follow the damn thing!
As an aside, let's face it, the plot of this movie is Mass Effect 1. They even both have a traitorious secondary villain that has the exact same motive in both.
Also, I do need to stress how much better the action was in this movie, especially compared to RotF's. Here (and this includes the human focused sequences) it was well-choreographed, well-shot, made good use of 3D, and was easy to follow and distinguish who was who. The fight scenes and big action moments were pretty well edited. In the first one it was pagued by shaky cam and odd camera angles and close-ups, but at least it was well-choreographed. But RotF, my god were those action sequences bad. I do not even know where to begin. It was impossible to tell what was going on, it was just explosion after explosion with no choreography or anything that makes a good action scene. Hell, there the Decepticons were so hard to tell apart I though Megatron died a third of the way into the movie. Even the highway scene, which you cite as being more of the same, was worlds better in this one. I mean, you're entitled to your opinion, but my god, RotF's action scenes were horrid for me.
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^Poorly disguised anti-caps sig