The world of comics has certainly come a long way. The Comics Code Authority used to ban any LGBT-related characters. But since comic book publishers like Marvel and DC have dropped the code the number of respectable openly gay characters has increased with heroes like Batwoman, Northstar and Green Lantern Alan Scott proclaiming loud and proud their sexual orientations. However, another big step was taken in April 2013 with the release of Batgirl #19 and the revelation of the first openly transgender character in a mainstream comic book.
In Batgirl #19 Alysia Yeoh revealed to her roommate Barbara Gordon (a.k.a. Batgirl) that she was transgender. The decision to make Alysia transgender came from popular writer Gail Simone (Birds of Prey, Red Sonja). Simone was inspired to create the character when she was with Greg Rucka (co-creator of the openly-gay character Batwoman) at Wondercon and he was asked by a fan why there were fewer gay male characters than gay female characters. Rucka replied it would be a real sign of change for a gay male character to appear on a comic book cover and tacked on that it would be an even bigger step for a transgender character as well. Simone decided to make this a reality (though Alysia hasn't made the cover yet). She also mentions that Alysia is bisexual in order to help readers realize that gender identity and sexual orientation are two very different things. 
I love how diverse comic books have become but it surprised me that there hasn't been a mainstream transgender character until now. There have been transgender characters in mature titles and independent comics, and in some cases mainstream comics like DC and Marvel have played with gender-bending (usually involving some magical malfunction).
Something I also loved to see was how quickly Simone's idea was accepted. She spoke about how she had to run it by Dan Didio (chief-editor of DC Comics and suspected hater of Stephanie Brown). Simone figured she would have to defend her decision, but the only question Didio asked after she ran the idea by him was how the character would affect Batgirl's story. After answering, Didio green lit the project. I love to see the higher ups react to such ideas so positively because it means there is a great chance we will see more of those kinds of decisions in the future being green lighted.
But Simone doesn't plan on stopping with Alysia. She has announced that she will be revealing another transgender character in another book she is writing, though she has not yet announced who it will be. 
I was curious to see how people were reacting to the possibility of more transgender characters and I created a "Gender and Comics" survey. One question I asked was "Recently a transgender character was added to the DC universe. Would you like to see more transgender characters?" Out of 29 people, 15 (52%) said they would like to see more transgender characters, 3 (10%) said they wouldn't and 11 (38%) said they didn't care one way or the other. I had heard Simone's decision was being highly praised, but the results still surprised me.
Simone mentioned a great point about readers possible hesitation to accept a transgender character. Readers don't have a problem with diversity. They have a problem with how that diversity is presented. According to Simone, Alysia will be:
"a character, not a public service announcement ... being trans is just part of her story. If someone loved her before, and doesn't love her after, well — that's a shame, but we can't let that kind of thinking keep comics in the 1950s forever...
It's time for a trans hero in a mainstream comic. I think it's time to make that thing happen that Greg [Rucka] mentioned years ago. And it's going to happen ... I'm sure it's controversial on some level to some people, but honest to God, I just could not care less about that. If someone gets upset, so be it; there are a thousand other comics out there for those people."
Here are the official interior pages of Batgirl #19 in which Alysia Yeoh reveals to Barbara that she is gay:
