Since the Health Care legislation is very similar to past GOP proposals, why is everyone so upset?

It's all because of the party, not Obama. Republicans started pretty early on with a strategy to reject and defame any and all legislation and actions done by Obama to try and create a culture of dissent much like the Democrats did with Bush. The only difference is the Republicans demonize literally everything Obama does, from sitting down on his chair the wrong way to not wearing a flag pin to buying hamburgers.