That's on top of the problem of determining whether something is of reasonable quality (i.e. even does what it claims) from a few screenshots and likely-gamed reviews.
The problem gets worse the more complex the app... Yes, reviewer A might find it useful because some subset of the features work OK for them, but perhaps I just want one specific feature which they don't care about?
Without a demo I'm left with the size of the community around and app, and a general 'buzz', both of which make it harder to enter the market at all.