Hi, stranger :)
Sounds so logical now, but never thought of it that way. Admittedly, I'm strange when it comes to books; I usually read the flyleaf after I've skimmed over the first chapter(s) (from which I make a judgement call...), so the summaries, in my case, are more an afterthought. Which is probably reflected in how my writing is…
You... Hate... Writing... Summaries? <holds fingers in shape of cross...> Blasphemy!
I agree. The two important/interesting elements work and would certainly blend in with what BW said. The set target audience would, consequently, be determined by those; for example, a SciFi romance would likely have the romantic elements outright listed, allowing the intended audience to know. Not only that, but, because of the main themes and elements being presented, you have, hopefully, a hook in the summary that draw people in (...which is exactly what a summary should do and what BW said already...).
Obviously, something called Attack of the Killer Lizard Blob People From Planet 672 is not likely to have "Killer Lizard Blob People attack! Bob milks his cows. Joan walks the dog. It's an interesting day." (Although, I'm kind of interested, in a strange way, at why Bob and Joan are doing mundane things as Lizard people are attacking...)