How do you get into a new series? The easiest way for me is for it to cameo a character I already love. Even if it is just one issue, I will pick up the next 4 or 5 issues to see if the title is worthy of following. I overheard a guy in my LCS today saying cover art is what makes him add a new series to his montly reading.
When I worked in a comic book store this was one of the truest statements ever. This is why it always baffles me whenever any comic series keeps the villain (Or any character) a secret.
Two examples I can give. Years ago when Kazar had a series, by issue 8 sales were pretty low (I think 3?) and they revealed the villain (From issue 1) to be Thanos. On that fact alone, I could've sold 5 more copies a month.
Jack Monroe was revealed to be Scourge in Thunderbolts #75. He had been around for 2 years. I like Jack Monroe, I possibly would have purchased the title if I had known. But I didn't know until issue 75, and I didn't want to try and hunt down back issues.
But to answer your question. There is no one factor that will make me purchase a comic, but instead it is a mixture of everything people have been listing.
Creative team. Do I like them or not?
Characters in the book? Do I like them or not?
Are there characters I hate in the book? (Not liking a character is not necessarily the same as hating them.)
Do I like the concept?
Is it a guest star and do I think the combination of the two characters will make it worth buying? (Superman/Batman, Superman racing the Flash. Batman/ Impulse.)
I like Peter David, but I have no interest in X-Factor.
I like Justice League, but the new series has no characters I find interesting.
Avengers Arena. Team superheroes on an island killing one another. Not really something I'd like to read.