Should landing on a runway that has been closed by NOTAM be a violation of the FARs?
There's no specific rule against it, so whether it should be a violation is a matter of opinion. FWIW, AOPA's legal counsel believes it isn't a violation per se, but the FAA has still pursued action against airmen for doing it.

The 'obvious' regulation involved is 91.13:


  (a) Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to
  endanger the life or property of another.


The NOTAM is presumably there for a good reason and even if the runway seems clear and usable from the air, there may be hazards or poor surface conditions that you can't easily make out. As AOPA's counsel pointed out in the article above, the FAA apparently ignores the requirement to prove "endangerment" but the NTSB also seems to back them up on that, so as a pilot you probably have little chance of arguing successfully that a landing wasn't reckless.

A famous (infamous?) example of landing on a closed runway was Senator Inhofe in 2010. Afterwards he said that no one checks NOTAMs anyway and it just wasn't a big deal, despite there being workers and equipment on the runway at the time. As it happens, he's a member of the Senate Transportation Committee, which oversees the FAA, and he represents Oklahoma, where the FAA has its headquarters. He was required to do remedial training and that might or might not have been appropriate but personally I suspect that if I buzzed some construction workers and then told the FAA that I didn't care about NOTAMs, I'd be a lot more likely to get enforcement action.