There was a trans character in a recent Brazilian series. They took a male actor and transformed him so well into a female-looking person that some watchers mistook him for a woman. The team behind the series did a great job with him and his portrayal was excellent as well. The voice and the mannerisms were spot on.I can't really think of many lesbians on television. Like, actually interesting characters who aren't just incidental who also happen to be lesbian. There's also a really sad lack of trans characters, but I think at the moment that might be more of a good thing than a bad thing; people find it difficult to do gays, lesbians and bisexuals right, and I think most portrayals of trans characters would be quite bad. The only trans character I can really think of too is Alexis Meade from Ugy Betty. At the time I didn't think that was one especially bad, but maybe I'm wrong?
Personally I'd be more interested in seeing more gay male protagonists in stable relationships, or just as protagonists at all, but that's because I'm slightly biased (sorry!). It's not like there's a finite amount of quiltbag that can be on TV at any one time though, right?
It's difficult to see trans characters that aren't comic relief because you won't easily find suitable actors. A trans female really has to be played by a man to be convincing, but it's not easy to find male actors with androgynous bodies. It's much easier to create a trans male character because women are more adaptable.
You can have women play trans females (like Lisa Edelstein did IIRC) but for me it seems a little off.
Oh and the very reason I liked House was because it challenges our beliefs, all the time. House isn't pro anything, and is anti-everything. He goes by his own rationality.