Honestly, I think sitcoms are, by definition, not High Fantasy and never can be. High Fantasy is not comedy, though it may have comedic moments. Sabrina, Jeannie, and Bewitched don’t have the framework to stand up to the demands of High Fantasy.
I think Raymond Feist’s work is eminently adaptable to a wide TV audience.
Run in a genre similar to the Black Company novels, plenty of magic and room for the various non human races.
I’d agree, if it weren’t for Galactica. If, on the basis of the supremely dorky 1970’s series, they could create the coolest show on television, then D&D should be a cakewalk. If you make a good TV show and market it strongly, chances are you’ll find an audience.
That may have been right in the past, but high fantasy, as a genre, has come a long way over the past few decades. Writers like Martin, Kay, Wolfe, Donaldson and more have been writing what is clearly HF, but also contains very human characters in very human situations - not to mention lots and lots of sex and violence, which always works on TV.
I’d love to see more Steampunk Movies or TV shows… there’s so much potential there (as a genre) that just isn’t being realised, IMHO.
TNT did a miniseries for The Mists of Avalon. I wondered why they didn’t just make a full length series, covering all the books. Too big a budget, or too much backlash because of the Paganism?
I was always taught that low fantasies were the ones that involved girls, especially girls dressed up like harem girls. My favorite kind of fantasy, actually! 
I had forgotten about the early 80’s Robin Hood series, the one where the original
Robin actor quit and Sean Connery’s son replaced him, after which the strong arc the series
had going for it collapsed. I enjoyed parts of Highlander TS, but far too many recycled plots
and it doesn’t quite fit in with what I had in mind (tho certainly discuss other fantasy here
too if you like).
Iwonder, though…
What about Escaflowne, would that count?
It’s only 26 episodes, and it has a lot of steampunk trends, but for the most part I’d categorize it as high fantasy. The other caveat is that the main character, Hitomi, is a modern “real-world” girl who gets transported to the world where the story takes place, just as Lakai mentioned.
But if you open things up to animation, particularly anime (and thus, not being restricted to little kids adventure series), there’s quite a lot of high-fantasy tv series (in Japan at least).
PS. Yes, yes, I just wanted to pimp Escaflowne, but it’s soooo good! Also, I try to push anime on people whenever I can; I’m not heavily into it (for example, if I can’t see it without buying it first, I’m not going to bother), but there really is anime for every genre/interest/etc and I wish people would get over the hang-up about animation.
I always wonder why Lodoss never got a run on the Cartoon Network. They owned the rights to the OVA broadcast, IIRC - they put it on the Toonami Reactor web streaming site but never on TV.
Back to the OP, I simply think the problem is the geek stigma fantasy brings. LotR has made roads to change that, but until you can get the 18-35 male audience to not say ‘ewww, that’s the stuff those nerds I beat up in high school liked’, then you’re stuck. Plus I’d be afraid of it turning more Heavy Metal magazine-style than anything.
Possibly the very best for high fantasy TV you could get would be something quasi-accessible to the general public that has the recognizable trappings of children’s fantasy; the writing could be structured to the satiric and postmodern sensibilities of a 21st century fan and allude to other fantasy series; the action could contain the lewd and lusty imagery of a good sword and sorcery epic; all the while while appealing to fantasy fandom community, your all-important 18-35 male demographic, your preteen geeks and your hordes of little girls who lu-uu-rve unicorns.
One serial fantasy series that nicely fits this sprawling framework and has the adaptability for translation into other mediums would be Piers Anthony’s XANTH novels.
Piers’ writing for the novels is mostly excreable, but Xanth is a fun world, with a hundred funny and interesting concepts and hundreds of interesting characters. I could easily see this series adapted for cable television. A SPELL FOR CHAMELEON would make for a terrific half season-long 12-part adventure; the events of THE SOURCE OF MAGIC could wrap up season 1.
Hell, I’d watch just for the CASTLE ROOGNA story arc.
Askia–before you were half finished, I knew you’d mention Xanth.
And, before the series ever aired, I’d hate it.
Askia, I’ve never read any Xanth stories, but what you’re describing sounds almost exactly like Xena: Warrior Princess only with elves instead of centaur. That’s nice enough, I suppose, but it’s not exactly what we’re looking for here.
A note: Vin Diesel taught Dame Judy Dench to play D&D for the Chronicles of Riddick.
This amuses me endlessly.
Nope, you’d get centaurs in Xanth too.
Oh, and adolescent puns. 
Actually, now that i just read Terry Pratchett’s “Men at Arms,” on well on my way to nominating DISCWORLD. So neener, neener.
Xanth could work with the right screenwriters and actors: the built-in story arcs are epic enough, the characters are varied enough, the milieu is exotic enough. But yeah, otherwise we’re talking XENA or worse, HERCULES and that’s not good enough for me to waste my time on.
Well well. The SF channel has a new series called The Dresden Files. It is
modern fantasy not high fantasy, involving a “wizard who helps Chicago Police solve
inexplicable crimes and battle dark forces.” Comic relief comes in the form of Bob,
a mischievous spirit trapped in a skull. This might prove to be something to watch-
just hope it avoids the more obvious tropes we’ve seen in other “lone wolf against
the evil forces” shows (c.f. Angel or even Quantum Leap ).