Someone outside of the startup world asked me "what is YC" and my reply was "the accelerator that rejects you."
My impression from having applied, and from knowing founders at various stages (some of whom went through YC, others who were rejected at various stages, one rejected at interview 5+ times in a row!), is that there is such a high volume of applications now that the person you get matched with to review yours is a total shot in the dark. If it isn't someone who "gets" your industry, you'll kind of just be thrown to the back of their pile. There is a lot of serendipity to who advances. I have heard some insiders complain that their recommendations aren't really taken seriously.
YC was the only accelerator who rejected us. We did a different one (at a true "idea stage") and it went great. Make sure to apply to others like Techstars, Capital Factory, Masschallenge, Betaworks, Boost VC, the list goes on. If it's a numbers game now, then whether you get picked for any particular accelerator is probably more a function of the total number of applications to that accelerator than anything else.
If you're super early stage, I'd recommend trying to find out if there are any accelerators sponsored by your local government, they tend to have way less applications since they're only pulling from a local pool.
There is probably a value-add to YC besides just money, but there are literally hundreds of accelerators that have copied the model at this point. If you really have something, the venture community that YC taps will notice regardless. It's always better just to take the cash and build IMO.
