Romney wouldn't have even tried to pass universal health care, nor would he have tried to pass cap and trade. He wouldn't have tackled any Wallstreet reforms nor would he have started negotiations with Iran at all. He wouldn't have opened relations with Cuba, he was opposed to marriage equality, and he would have been even more lock step with Israel than Obama, who has done his best to navigate that morass. Beyond all of that, he would certainly not have nominated the same justices Obama did.
Furthermore, if you honestly think that Bush and Obama were "nearly indistinguishable", or that Clinton and Bush where "nearly indistinguishable", than you have not been following politics with any degree of seriousness or you are being willingly hyperbolic.
This kind of sentiment is what scares me about 2016. If you want change, you need to first and foremost vote progressives in in Congressional and Senatorial elections and take control of congress. Couple that with a competent leader in the White House who has the political acumen to navigate the landscape of American politics and you will get SOME change (note some because politics is messy). Sitting out one of the most crucial elections because "all politicians are the same lol" is a position born of willful ignorance.
Honestly, Sanders will do much more good in the Senate just like Elizabeth Warren will. Having a powerful liberal block in Congress will do more good than having a President Sanders with a Tea Party controlled congress with ZERO ability to compromise because they are so ideologically divergent.