While I think that we should "pull our punches" to some degree when discussing the First Ladies...I do think that anything they do in the public sphere is up for comment, critique, and downright criticism.
I feel that the disgusting comments regarding Laura Bush's teenage and college years I read on this site were unbelievably out of line, just like I view the rude comments about Michelle Obama's body, hair, attitude, etc.
I do think, however, that comments regarding Michelle Obama's "Just Move" campaign and decisions she makes that relate to that are fair game. I feel that trips she takes are fair game. I feel that when she makes a comment that reflects (or may reflect her opinion) regarding issues that I care about...she's fair game. She is the First Lady. She represents our nation. She chose that lifestyle by marrying a man who chose to run successfully for President of this nation.
I think that, for the most part, it is fairly easy to tell the difference between legitimate criticism and partisan meanness. I think, however, that each side tends to get a bit more defensive of "their" First Lady...which is, perhaps sadly, natural.