Jumping in late to this thread...

Battlestar Gallactica: I'm really enjoying this show for all the same reasons cited by others here and elsewhere. I noticed that they're reairing the pilot on BravoHD soon, so I programmed the TiVo to grab it. Normally, I don't go back and rewatch things like this, but, well, to see BG in full HD glory... that's worth watching it again. I'm just hoping that BravoHD carries the rest of the episodes; it will really benefit from the HD treatment.

Alias: I'm still crunching my way through the season 1 DVDs and loving it. I also started watching the current season and I concur with the "one happy family going out on missions" comments. It just doesn't have any of the tension of the first season, where you don't really know who you're supposed to trust and you really feel that Sidney has no way she's going to get out of this predicament. The new opening credits also now tell the story. Watch this show so you can see our stunning lead actress dress differently every week!

Carnivale: One of my favorite shows. I also concur with Bitt's comment that it's just like a DC Vertigo comic. It's gritty. It's dreamy. Every character, however minor, has a complex back-story that is slowly, slowly being revealed. The story weaves in and out with the actual history of the Great Depression in an almost believable fashion. (Trying to stay spoiler free, but...) we also have this epic good-vs-evil tension, where neither character fits the archetypical role in the way you'd see in a more traditional story. And, again, we don't really know the full nature of what makes the good character good and the evil character evil. We (and the characters themselves) have only seen glimpses of their power. And, Carnivale is just stunningly beautiful in HD.

Entourage: Another great HBO show, following the only barely fictionalized story of a recent Hollywood star (Mark Wahlburg) and his New Jersey buddies who he dragged along to be his "posse". HBO's supposed to be working on a second season. I can't wait.

Iron Chef America: I was sucked into the original Japanese version, and I also love Alton Brown's Good Eats. This has both of them together. Works great for me.

24: I've watched this completely out of order. I watched the second season live and really got into it. Then I got the first-season DVDs and puttered out about half-way through. Likewise, I started watching the third season and it just wasn't doing much for me, so I gave up on it. Now, I'm watching the current (fourth) season and it's got me hooked again (and, likewise, wondering how all the people got shuffled around to their current positions).

Frontline / Nova / Nature: I'm a sucker for good PBS shows, and they rock in HD.

My wife is mostly enamored of the Law & Order series, the CSI series, and some other closely related clones (Cold Case, Missing Person, etc.). These just don't really cut it for me. I particularly hate the often abrupt and unsatisfying endings.