What about the design? The user interface is important obviously, do you know design 'well enough' to make a passable interface or do you work with a designer?
Also, there are quite a few themes/skins/ui-kits for bootstrap and other front-end frameworks, that are worth taking a look at if you don't have a designer to work with. They won't compare to having a real designer on your team, but they're better than nothing... you can get something passable up and running that isn't embarrassingly ugly.
But, this only applies if you're building products whose primary value proposition (and product market fit) has little to do with the design (think Craigslist).