I agree with you Scott in the sense that the term "Bad Beat" is vastly overused. Like you said, a lot of hands discussed on poker forums as being "bad beats" aren't really that bad at all. I'm not really sure how to define a Bad Beat though.
I played a low buy in heads up sit and go today at PokerTime with my free $10. I had about 1100 chips to my opponents 900 when i saw JJ on the button. My opponent had been wildly going all in the past couple hands and showing his or her weak cards, so I simply called instead of raising - expecting him to go all in - which he did. So i called. He had Q2 offsuit - I was happy to see a bad hand, but knew that i was a long way from taking him out. Of course the river turned out to be a Q, and I was down to about 200 in chips. Now this sucked, and i wasn't happy - but it wasn't THAT bad of a beat. He had an over card, and he happened to hit it. The real kicker came a couple hands later. I managed to double up through him once, and had ~400 in chips now, when i saw JJ again. Not one to be superstitious, i played them again anyways (i regret that decision now
) He called my 350 chip raise with 10 7 off suit. I was looking great as he didn't even have an over card. The flop came KQ7 off suit - pairing his 7s and making me sweat a little. The turn was a blank, and the river was another 7 giving him trips to beat my jacks. Needless to say, I wasn't very happy. I consider this one to be much worse of a beat. He willingly put in about 1/4 of his stack with two unsuited under cards (he didn't know that, but it wasn't a good hand). I was a huge favourite preflop, and even postflop - and got screwed on the river. That, to me, was a bad beat.
Simply defining a bad beat isn't easy - i really think it depends on the case. I can't really think of any set "guidelines" for determining whether a beat was bad or not. Unless you look at percentages and say that if you are a 90% favourite, and lose - that's a bad beat. SOmething to that effect