I recently applied to YC with my startup Pindragon (a location-centric social communications app). Pindragon is not launched so I have zero traction, but I do have the it nearly ready and young people that are generally the early adopters like it. I didn't get selected but what I like about YC in order of precedence:
1) 120,000 USD for 7% of a pre-traction company seems reasonable given my research on Pitchbook, Crunchbase and elsewhere of seed stage valuations. I like that they created and use a SAFE and they don't tell you what to do with the money they invest. That is a demonstration of trust in the teams they've chosen. I kind of like that they don't provide any office space, it's up to you to find your own. I like that to get mentoring or advice one just schedules a time slot on their calendar. And the weekly dinners sounds like a nice way to get comfortably acquainted.
2) One is going to be fast-tracked to more rounds of funding if there is strong traction and retention. Their introductions will make this happen quicker. There are many people doing similar startups and those who get funded faster are at a strong advantage.
3) Access to talent. I feel like they get so many applications that there would be some that applied who's idea isn't great, but their talent is. So for example, I thought maybe I could find another great co-founder or attract a great co-founder while being in YC. (Background: My friend and I co-founded Pindragon but our business dynamic was not working as things proceeded so he gave up half his shares and resigned. The irony is that it was his idea, but I was the one doing all the coding so it made sense that I was the one to continue. We remain personal friends still.)
4) Mentoring. I don't think I need mentoring, but I do see a lot of value in having advice and a sounding board. There are many times I wish I had people who understood startups to talk to and ask their opinions. That said it is difficult to find really good advisors.
What would prevent me from joining an accelerator is the following:
1) If they are only providing mentoring then I wouldn't join. The Accelerator must make an investment of at least 100,000 USD at a reasonable valuation. I think YC's 7% for 120,000 USD is about as low as I would go. I'm soon to launch and if I have strong traction and retention (>100% organic growth MOM for the first few months) an Accelerator will be off the table, if it is less than that then it will be still of interest. Since I have young people that have seen Pindragon asking me when it will release I'm thinking that I'll have strong traction, but the proof will be whatever is the real case. (Another aside, I plan to target SF Bay Area first precisely because I'll get on the radar of VCs quicker. My son is at SF State University, and I've been building my connections there. I live in Santa Barbara so I will also focus on it.)