Biggest obstacle to taxing the rich seems to be the voters. For example, here in Washington State, the voters sent a resounding 63.9 % yes to an initiative that would require 2/3rds vote in legislature for any tax increase. I-1185. Anti tax sentiment was still strong at the ballot box even though that same election was considered a liberal victory for legalized pot and gay marriage. The two thirds requirement has since been tossed out as unconstitutional by the Washington State Supreme Court, but seems like it's hard to get new taxes, on the rich or anyone else past the voters. California voters did pass a state tax increase in 2012, but such vote is still rare. If we really want to tax the rich, we have a lot of educating to do at the grass roots level. Convincing voters to vote for tax measures that can tax the rich.
Ballotpedia article on I-1185.