Discussed this a bit in the official thread.
I think the two main factors are the aesthetics element and the age one. Personally I find Hussey a wonderful batsmen to watch, his shot selection is absolutely amazing at times. As I said in the other thread, you can watch the guy leave five deliveries outside off stump and choose to play at the sixth, and he'll hit it perfectly through the gap. He so rarely makes a poor choice of what delivery to play, or plays at a delivery from a poor position or really does anything else wrong that it's actually shocking when he does.
So while that might appeal to the purist viewer, he doesn't exactly go out and dominate attacks like a Ponting or a Tendulkar or whoever. Even Kallis, who is also fairly stoic, has a wider range of shots and when set, will play aggressively at a larger number of deliveries. Hussey, unless runs are needed quickly, is perfectly content to wait as long as it takes for the right ball and then put it away. He obviously has a big "extra gear" he can use if necessary, but that's not his normal play.
The age element also comes into it, especially for your more discerning cricket viewer who isn't turned off by the way he bats. There is something vaguely surreal about Hussey's success, like Richard implied, and it's hard to really follow in some respects. Obviously most of us expected the guy to be a success in test cricket when he arrived in the team. He'd been arguably the best ODI batsman in the world in the past year or so before that, freakishly prolific, and had years of FC experience. Nevertheless, what Hussey's done up to this point is not simply a good start. Nobody has ever played so many matches and maintained such an average except for Bradman, and the longer he keeps it up, the less of an anomaly it seems.
To keep it in perspective though, Hussey's record in the time he has played isn't that far ahead of the pack. Guys like Sangakkara, Chanderpaul, Ponting and Kallis would have similar averages over the period, and in fact someone like Ponting has averaged in the high 60s for more than five years now, so of course, Hussey's success isn't unique in the modern game. What is unique is that he took no time to settle in and there have been no blemishes. It's as though you took Ponting and removed his entire career from '96 to '01 or so, and just left the period where he's been dominating attacks all over the world. If you did that, you'd have a guy with well over 5,000 test runs and an average of almost 70, and probably have almost universal agreement that he's the best batsman since Bradman.
What holds Ponting back is that it's viewed as a "purple patch". With Hussey it can't be, but because he made his debut at 30ish, people tend to artificially add that "learning phase" in and bring him back to a more normal level.