I tried to address this question a bit when announcing this site on BoardGameGeek. My conclusion was that we're more complimentary to BGG, rather than competing with it, though obviously there's a certain amount of competing for people's attention and time. To quote myself:
StackExchange is a question and answer system which has grown out of the very popular https://stackoverflow.com/, a Q&A site for professional programmers. It has several advantages over other forums or Q&A systems. Unlike forums, you don't have to wade through pages of responses to find the actual answer to a question; answers voted up will move to the top, and the questioner can accept an answer which will become the top one. Unlike other generic Q&A sites, StackExchange sites have specific communities devoted to the topic in question, not just a lot of people answering questions on everything. And unlike most sites of the sort, it also involves wiki-style community editing, in which users with enough reputation can edit questions and answers to provide clarification, fix small errors, or clean up formatting or tagging.
BoardGameGeek is a great resource, and the Board and Card Games StackExchange is complementary to it, I think. BoardGameGeek has a lot of information on it, and sometimes people might not be able to find the needle they're looking for in the haystack of BGG.
First of all, yes, BoardGameGeek has some definite advantages in the size of the existing community, and the integration of its game database. The first is one that can hopefully be improved as this community grows; likewise, the issue you mention about designers already being on BGG could be addressed by getting designers to answer questions here as well.
But that still leave the fact that in the meantime, as we grow our community, we need to be able to say what differentiates us. I think there are few things:
- The biggest benefit is the actual StackExchange system with questions, answers, voting, and editing. On BoardGameGeek, if you want to ask a rules or strategy question, you have to do it in a forum format. This means that you ask a question, and then there may be several replies of varying quality that are hard to sort through. In some cases, you may get several pages of replies, at which point finding anything is quite hard, and the SEO will be totally broken because the question and answer will be on different pages. On StackExchange, the best replies will be voted to the top; the one that actually answers the question will be at the top and prominently marked. Small errors, or typographical or formatting issues, can be fixed by anyone with enough rep to edit other people's answers (or left as a comment that will be displayed immediately below the question or answer), instead of having to keep reading through the thread to find the one bit of errata that someone else posted.
- The simplicity and focus of the site. BoardGameGeek is very complex and busy, with lots of stuff going on. It can be hard to figure out how to ask a question, or where to look (a few of my more poplar answers here have simply been instructions for how to use BoardGameGeek effectively). StackExchange, in contrast, is relatively simple and focused on question and answers. I've pointed out BoardGameGeek to people and heard "ow, I can't even figure out where to start looking."
- Better SEO. This is related to the above; the simplicity and focus on SE sites, as well as the emphasis on quality content through voting, helps it achieve better Google rankings. There have been questions that are answered on BoardGameGeek, but a simple search didn't answer it; I had to try adding "site:boardgamegeek.com" to my search in order to find the answer, and filter through a lot of irrelevant results.
- The Creative Commons license on all of the content is another reason that I prefer to use SE to BGG. I used to contribute fairly heavily to BGG, but BGG and the authors of any content retain sole rights to any of the content posted there. Thus, BGG is allowed to use and profit off of my content as they see fit, while I'm not allowed to use the database or anyone else's content for any other purposes. This makes it hard to, say, create a read-only fork of BGG with a better interface, which could be quite useful and by competing with BGG inspire them to clean their interface up a bit. BGG is a collaboratively edited resource, but pretty much wholly owned by a few individuals, while all of the content in SE is available for anyone to use as they see fit (within the bounds of the license).
Finally, I disagree with your point about the focus being broader here than at BGG. I think that what's on topic at each is pretty much equivalent. BGG has Risk, Monopoly, and Trivial Pursuit pages, with forums that people discuss them, which can show up on your front page. There may be a slightly different proportion of questions on certain games here vs at BGG, but that's going to be the case anywhere; I don't think that in the long run, there will be much more discussion of Trivial Pursuit here than on BGG, especially if we succeed in getting more serious gamers involved here. And if there are games you're particularly interested in or don't want to hear about, add them to your "interesting tags" or "ignored tags" lists.