That a lot of backers have real fun in the alpha ?
I have lots of fun in ED, but to listen to many, ED is a boring space trucking simulator devoid of fun.
Its not really a subjective pointer as to why someone should spend money on something.
The best way is to ask questions along the lines of:
Do you like spaceship games?
Do you like lots of attention to detail?
etc...
And build from there, while of course, also pointing out the limitations and bugginess of the current state of the game.
Then you let others decide for themselves whether they think its worth paying for.
I always appreciate those on reddit who, in response to questions about whether someone should get the game, take the effort to tell people about the alpha state, the bugginess, etc, while at the same time saying what they like about the game. Those people i'll upvote.
When people start banging on about how its already got more content than any AAA game out there (as they learned at Uncle Chrissy's knee), that's when i take umbrage.