Proposal: Biblical Hermeneutics looks like it's explicitly about Christianity, and I believe it would be best merged with the Christianity proposal.
Although honestly, I'd probably prefer to see a more generalized Religion SE site.
Being the one who proposed the Hermeneutics site, I can't immediately think of any reason not to merge it with Christianity. Just as long as people understand that Hermeneutical questions deal with objective reading and not traditions or extra-canonical teachings.
I think the sites should not be merged
With the current definition limiting it to Christian Scriptures, the site is of interest to Christians and Jews alike.
If merged with Christianity:
Disclaimer: I happen to be Jewish
As someone involved in planning sites for virtual communities (and a Christian), I think it's more likely for a more-specific site on Biblical hermeneutics to succeed than a more general one about Christianity.
One profound difference between Stackoverflow (Programming) and a site about Christianity is the great deal of advocacy around Christian topics. We could end up being a "walled garden" of a few bickering users from various viewpoints. As an Anglican, I see how, e.g., at Wikipedia, many Anglican articles are of very low quality, due largely to the intense advocacy around issues in the Communion around sex issues. As a result, many of the Wikipedia articles on contemporary Anglican issues are highly misleading, and those who are well-informed about contemporary Anglican issues don't seem to be contributing much.
On the other hand, Wikipedia does have quite a few articles which are of good quality for Christianity in general; so my remark here must be qualified.
An advantage of Biblical hermeneutics is we have some good potential guidelines. E.g., "We are discussing primarily what the Bible says, and not primarily whether or not it is true." Atheists and persons from other religions are warmly encouraged to take part; but on this limited issue (and thus, temporarily putting aside the question: "is the Bible relevant to us today, should we take it seriously?").
If we are to succeed with the site, we need to draw quite a few well-informed "experts" who have been dealing with these topics for decades, and are willing to invest time in writing answers. I think we're more likely to attract them if the topic here is narrower; and that we're more likely to have bickering-type threads, dissuading everyone except for the die-hard activists.
It's also worth having a look at what user @mfg had to say in his participation on the Atheism Q&A - he noted that people in Atheism were often distracted by commenting a lot on hot-button questions: "Without clear, concise questions that yield answers, and people participating via answers rather than comments (where bad responses can be voted on their merits), the site will stall half way through beta" (from Christianity: Are Rational Questions Allowed?). I'm not so worried about not having enough questions ... I'm more worried about the quality of the questions. I think this might result in a site with enough traffic to get through the beta, but where few are really happy about the resulting site. For a topic as big as Christianity ... it's worth having this site done well, a good resource for all parties concerned ("for," "against," and neutral).
It's possible that a general Christianity site could succeed, at a later stage - but only after we have a good, quality site on Biblical hermeneutics, which helps establish our reputation amongst Christians who are both articulate and informed. If we don't first establish ourselves somehow, I think we'd likely end up becoming something like christianity.about.com - not very significant, with mostly rather polarized views that aren't interacting very much (my rather uninformed perception of christianity.about.com).
What Amichai here says is also interesting - that Biblical hermeneutics would be interesting to Jewish users. The Bible is one of the most important instruments of ecumenism, holding Trinitarian Christian churches together - all Trinitarian Christian churches uphold a high sense of its authority (e.g., Anglicans uphold it as the rule and ultimate standard of faith). Only in the last 50 years or so have different churches been rather intensely engaging one another on their understandings of scripture, which has generally had positive effects on ecumenism and Christian unity. E.g., we see Catholic theologians intensely interested in N.T. Wright.
As someone who has been informally teaching Biblical hermeneutics for many years (by leading Bible studies where the issue of interpretation is always a high priority, where we sometimes discuss the influences on hermeneutics of people like Schleiermacher and Gadamer), when I visit Web 2.0-style sites with lots of user-submitted content, I frequently think: "There are quite a few people here who don't know the topic as well as they think they do," and I go somewhere else. Having a more specific topic here, I think, would help cultivate the better-informed users, and provide a better basis for voting on questions and answers.
So I think it makes sense that the first site StackExchange does on Christianity would center on specific discussion of the Bible. This is, after all, when it comes to Christian theology and practice, where it all comes from (tradition and reason of course given their appropriate roles - but then again, in interaction with Scripture). If we are able to attract users who are highly knowledgeable about the Scripture - probably first amongst those who believe in the authority of Scripture - we may also eventually be able to attract quality interpreters of the Bible who do not believe in the authority of Scripture, and are able to provide a meaningful and well-reasoned counter-point.
I lead a small Bible study that uses the inductive method and I find that non-Christians, marginal Christians, and even skeptics are interested in understanding what the Bible says quite apart from what Christians over the years believe that it says. In other words, people often search for meaning in the scriptures before they search for meaning in the Church if ever. (I should point out that Church tradition and the Bible usually agree.)
Biblical Hermeneutics, as I've come to understand, turns out to be a very technical topic not dissimilar from programming (at least the debugging side) or historical research. As James pointed out, serious study topics tend to work out better for Q&A sites than broad topics. There's a stark difference between, for instance, blogs that cover textual criticism on the one hand, and general Christianity on the other. I suspect that a Christianity site would break down into sloppy appeals to authority (which no two people agree on), random flames (from all sides), non-sequitur comments, and so on. No thanks!
Now I know this style will drive away a lot of people. (My Bible study stays small for a reason.) Even so, those who stick around will be more actively engaged asking and answering good questions than in some sort of free-for-all. Personally, I look forward to hearing dissenting viewpoints since often they either a) refine the position I already hold, or b) bring me around to another, better understanding of a passage. I'd love to ask questions on a Hermeneutics site in addition to answering them.
With the current definition limiting it to Christian Scriptures, it should. But if the definition were changed, it could involve other faiths, such as Judaism and Islam, in the discussions. Then it wouldn't be part of Christianity.