There's an excellent little piece on why the firing of the US District Attorneys is such a big deal:
Given the amount of attention we've given to the US Attorney Purge, there's been no end of right-wing nutjobs who've written in asking just what the big deal is. In most cases, these are just attacks dressed up as questions. And I do my best -- not always successfully -- to ignore them. But interspersed in that mess of emails are a few who seem to be asking, genuinely, what the big deal is. Perhaps they're critics of the president or conservatives who genuinely don't see it. So here's how I'd answer that question.
For all the intensity and hostility awash in our politics, there are some lines we just assume aren't going to be crossed, lines that are so basic that the civil compact itself can't easily survive if they're not respected.
...
So what you have here is this basic line being breached. But not only that. What is equally threatening is the systematic nature of the offense. This isn't one US Attorney out to get Democrats or one rogue senator trying to monkey around with the justice system. The same thing happened in Washington state and New Mexico -- with the same sort of complaints being received and acted upon at the White House and the Department of Justice. Indeed, there appears to have been a whole process in place to root out prosecutors who wouldn't prostitute their offices for partisan goals.
We all understand that politics and the law aren't two hermetically sealed domains. And we understand that partisanship may come into play at the margins. But we expect it to be the exception to the rule and a rare one. But here it appears to have become the rule rather than the exception, a systematic effort at the highest levels to hijack the Justice Department and use it to advance the interest of one party over the other by use of selective prosecution.
http://talkingpointsmemo.com/