Being immersed in the Twilight fanfiction community AND the Batman fanfiction/fanart community, I have noticed some interesting similarities and differences.
Both Twific and Batfic authors expand upon characters and storylines, proposing situations and relationships that were not addressed in canon. That's about where the similarities end.
To make huge generalizations about the differences...
Twific authors seem to write stories that are serials, updating chapter by chapter. Time between chapters varies greatly but the majority of stories never complete. The majority of Batfic tends to be oneshots. Because Batman is a long running fandom, the fic series that I've come across so far have all been complete.
A lot of the great Batfic I've come across (and I can only speak to the slash side of things) has been written several years ago. Many of the authors are no longer active or have moved onto other fandoms, so there's not much interaction between reader and author. There are, of course, exceptions. I suspect there will be more activity around when the new Batman movie comes out.
Twific has a vibrant community that continues to thrive. The connection between writers and readers is even more enhanced by the accessibility that Twitter offers, and the array of blogs, contests and awards sites (and podcasts) that support the community. The sheer volume and diversity of Twific stories is also incomparable.
The general fandom's response to slash...
In superhero fandoms that have been predominantly teen (or lifelong teen-in-spirit) boys for so long, slash is more subversive. Slash stories and art are often created by girls or women, groups who have not historically been seen as equal players in this fandom.
I've seen male-on-male superhero slash dismissed as an annoying or even offensive thing that silly fangirls do. I don't often encounter that douchery because I ship slash, and I run in slashy circles.
Characters that are regular humans in regular situations...
In Twific, it often seems as though there are more All Human stories and readers than AU vampire stories. It's actually something I have found so interesting about the Twilight fandom -- the use of the Twilight characters in completely non-vampire, non-Twilight-related situations, and characterizations that are so often OOC. The AH phemonenon has led to a community of women writers that has used Twilight as a jumping off point, but is by no means tied to it. A community which has more to do with the creative process and reader feedback than it does with the fleshing out of Stephenie Myers' Twilight story world. I have often wondered if this is unique to Twilight. If other fandoms have such a huge percentage of stories that use the characters in name only.
I haven't encountered much of an All Human equivalent in Batfic. There's certainly the potential for AH in superhero fic: a superhero character without superhero powers, much like Twific writers use vampire characters as regular old humans. I think the closest thing is Batman and the Batfamily of characters who have no super-human abilities, yet train themselves to carry out superheroic missions. But that's canon, not a fanfic invention like it is in Twific. I haven't come across any fic where Bruce Wayne is anyone other than the Bruce Wayne/Batman crusading persona. HOLY SHIT what I wouldn't give to read some Bruce Wayne as Beautiful Bastard. I just pictured Bruce Wayne instead of Edward in MoTU and nearly fainted. *fans self*
There are so many similarities between Edward and Bruce Wayne... broody, self-blaming, assumers of guilt, access to wealth, extremely insightful readers of people who at times seem oblivious to the emotional reality of a situation, one is the World's Greatest Detective and the other has vampire skills of detection.
But I've never read a Bruce that was OOC, aside from behavior brought on by sex pollen (a weapon of Poison Ivy and a delicious Batfic fandom cliche).
Picturing various AH/OOC Edwards as Bruce Wayne has now inspired me to search for it.
My caveat to this post is that I've been involved in the Twific community for almost 2 years now, whereas I've only gotten into the Batfic community over the past 6 months. So make of that what you will.