I agree with you, the "battle" between Sideous and the three Jedi + Mace Windu was pretty awful.
I guess we're supposed to assume that he used the Dark Side to basically just turn off the Force sense of the Jedi in the room, making them absolutely horrible.. and only Mace Windu was able to shrug it off in time to stay alive. Ian McDiarmond actually had the least amount of sword-training of any of the actors on set (don't know if they used a standin, just CG at various points like during his flips). As a result, they had to incorporate his silly style (not that the other styles aren't silly, but his was the most amateurish) into something that was supposed to look dangerous.
It just didn't look very convincing on camera. It shouldn't require fanboy interpretation to make the scene make sense. Either make him an awesome fighter who overwhelms them with pure skill, or make the fight longer so it's at least convincing. If he's supposed to just be paralyzing them with fear so they can't fight, show that...
I agree about the Battle Droids as well. Here we had a potential for a fearsome army. Perhaps Lucas wanted to show us that droids were just silly in combat (but then why create Grievous and the high casualties at Geonosis?) and therefore clones replaced them. The Jedi were shown to be powerful by easily destroying them in Episode I, but perhaps if they had shown them being more successful against non-Jedi, that would boost both their and the Jedi's reputation (rather than making them appear so weak anybody could easily destroy them, making them weaker than normal troops).
The OT stuff is only explained by saying that Vader convinced Tarkin to let the Rebels escape to lead them to their secret base. But remember that this can't have been the original plan, so at the start at least, Luke & co. were just darn lucky/skillful!