Obviously I can only theorize why someone else buys a comics, but for me a comic has to have more then one element to draw me in. Also I have a tendency to buy more then I really care about in general for a variety of reasons, not the least that in order for me to be a critic I feel I have to see a lot of the material. As with most people who are immersed in an area, they have a tendency to like stuff that is further away from the center of that medium. Movie and TV critics seem to like the offbeat, because they are burnt out on the stuff the general public may view as entertaining. I believe this is true with comics also. The people who read the most will love certain books, that most people ignore or don't like.
One element that I think is important is the character that the book is about. When I started reading comics (age 5 or 6) I never cared who was writing it or drawing it I wanted the next issue of the Fantastic Four, Amazing Spider-Man or Batman. I never really paid attention to who was writing it. It is still a strong pull for me today and impacts my opinion on a book. I like the Martian Manhunter and will grade the book easier then if the series was about Booster Gold. If you don't like a character it is going to be harder to even try a book about the character.
The writer is a big element for me today. I'm more willing to buy a new series by Grant Morrison then I am by Brian Bendis. Still if Bendis is writing a book about a character I like with a good artist I would try it out. Morrison could be writing about a character I don't care about and I will probably still try it.
The artist can be a huge element. In fact with Bryan Hitch and John Cassady I will buy the deluxe format hard cover of Ultimates Volume 2 and Astonishing X-Men even though I think both series are just okay. Their artwork is so nice that it makes a so-so story worth the read. The counter point to this that even a great story poorly depicted is hard to follow.
Concept is a huge element and the biggest draw for me with many of the independent comics. Reading the premise for Wasteland drew me to trying out that series. I enjoy post-apocalyptic stories and Wasteland is 100 years after the world has been destroyed by the "big wet". So far we have to assume it was a major ecological disaster that Al Gore has blathered on about forever. Still that concept made me want to buy the book and try it out.
On time publishing. This was more of an issue when I read fewer comics. At the insane number of books I'm getting monthly now I hardly notice an issue not coming out. When I was only getting around 30-40 books a month and I'm waiting for the next issue, a two or three month lag was unacceptable. Long delays in publishing between issues either make me drop the book or just get the trade once it is done.
Investment. This is an element that I think is a big one. Not only money but time. I think the slow publication and long gaps between issues hurt many independent books and then when they go to trade a $15 investment for a totally unknown product is daunting. X-Isle is a fun little mini-series (5 issues) that is just now having issue #3 come out and it has been months between issues. It is a $3 (maybe $4) an issue book. The delay in publishing would make many people fall off this series and makes the retailer order less so you don't even have a chance of picking it up. Then sometimes it may not even be finished.
Being in the know. Marvel does a great job of making certain books key to having a feel for what is happening in the overall universe and certain books being core books. In other words if you follow the Marvel Universe you have to follow a certain title. Recently I believe Marvel has made New Avengers that book and used it to build into Civil War.
Obviously any combination of the above elements make each person decision a reason to buy or not buy a comics. A book about Sugar and Spike written by Bruce Jones and drawn by Frank Quietly done as a four issue $6.95 deluxe book would never sell. But a mini-series about Bruce Wayne and Diane Prince being lost in a dimensional warp and stuck together for a few years written by Grant Morrison and drawn by George Perez for $3 for four issues would be an immediate add to my list. A real example is Fell. This was an immediate sign up for when I heard Warren Ellis as writer, one and done stories, a detective in a run down town and a $2 price tag.
It is important for the strength of the industry that people re-evaluate why they buy what they buy and try to expand their horizons. I believe that various factors makes it hard for fans to decide and buy Human Target over New Avengers. New Avengers comes out monthly, has a writer many people like, name artists, has major characters that a lot of people love and has the feel of being a core title to know what is happening in the Marvel Universe. Human Target has an obscure character, a less known writer and if you miss this book you only miss what happened to Christopher Chance. It is maddening because a book like the Human Target was well written, drawn and had wonderful literate themes about identity.
Hopefully people can re-evaluate why they buy a book and try to look around and try something different as opposed to buying a spin-off of Avengers or JLA.