Cheaper -- you can bootstrap really easily here compared to other places even nearby (DC, NYC, BOS).
Community -- it is really a nice community where you can really reach anyone really quickly and they're willing to help.
Would you add anything?
I'll second community as well. Really helpful and approachable people in the startup community of Philly. I've never spoken to you, but have read that you are among the most approachable in the area.
I might add the available feeder system of colleges in the region - Penn being obvious, but some less obvious schools like Lehigh and Villanova that Philly firms can tap for talent. Unfortunately those students often leave Philly, but as they get more options to say they will do so.
I think the area is still in need of one or two big name companies to keep talent in the area. A larger dev satellite office of a Google/FB or even Netflix type would help keep those grads in the area after graduation, and at some point they may have grown deeper roots (families, kids in school) that might keep them in the area. I'm starting to see more and more local engineers moving west, as they feel they are "outgrowing" the area. I think a company with a good employer brand (as it relates to software) would come in and be able to hire a ton of great talent.
Thanks for answering, and best to your continued success.