Today in section A4 of the republican
They had an article about the Mcdonald vs Chicago supreme court case and at the end of the article they cite Sotomayr (SP?) as saying "Would you be happy if we incorporated it and said reasonable regulation is part of the incorporation?". But what is reasonable? here in MA we have a long list of regulations/restrictions that MA has deemed reasonable, now whats to stop MA after this ruling which hopefully will be in Mcdonald's sides favor from handing out more and more regulation that they deem as being reasonable
"Reasonable" is something that is very subjective, you are right in that regard, as for what defines it or how it would be defined, that's something that will take many legal cases to hammer out.
Each regulation in a state that someone sees as "unreasonable" can be challenged in the state courts, and the judge presiding over that case will then have the opportunity to look at the McDonald case majority opinion and decide if the regulation or law being discussed violates some portion of that decision and infringes on a person's rights.
So you may never get a nationally uniform definition of "reasonable", reasonable to Mass is unreasonable in NH, so you're more than likely going to have varying degrees of reasonable from state to state.
Don't get me wrong, once the first few cases are heard with regards to "reasonable", you're going to have precedence in the preceding cases that may help to clarify the matter legally down the road, for better or worse.