If your user-acquisition strategy is 'cold-approaching' people (ie asking people to try it who don't know you), the app should look polished. It doesn't have to look like a billion-dollar company created it, but it has to look polished. Facebook's MVP looks a little less stylish, but Zuck wasn't cold-approaching the campus; he was already well-known. And even Facebook is using some CSS and a custom heading to make things look somewhat decent. And then when he was expanding to other campuses, they had the advantage of being able to say "It's this new thing that everyone at Harvard is using." IIRC when I first heard about Facebook, I also heard that it had started at Harvard.
What "polished" means:
Fonts:
Custom fonts
Shadows
Inner shadows / gradients
Sans-serif fonts
Instagram's non-cursive font is serifed but it's so blocky that it's a serifed font that looks like a san-serif font.
Pictures:
Stock-photo-level pictures
Buttons:
Bevels
Inner / outer shadows
Inner gradients
Transparency
Colors:
A modest variety of colors. Maybe 2-3 colors other than white.
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