Tip number one in marketing: make it easy for the customer.

I'm not going through the hassle of installing something if I don't already know I want or need it. No screenshots -> "Next!"

It's the same effect of all the (no doubt wonderful) tools that begin their website with "Foo is the next Bar, now at version 3.1, here is the changelog:...", without ever mentioning what Foo even is. So many customer/users lost through bad marketing.

Yes, absolutely. There's nothing more annoying than theming systems that don't allow an instant preview. Nobody wants to install a whole new theme just to decide if they even like it in the first place.

Not to mention, actually trying the theme might be completely impossible given the circumstances (e.g. browsing on an iPad).