To me, here's what makes a great comic:
• A good artist. I'll prefer something by Jim Lee over Rob Liefeld any day. Good art just helps make a good read.
• Dialogue that works with the characters. A lot of the 60s Marvel stuff that was amazing in concept was killed by crappy conversations between characters. The readers learns what is going on through the characters, so it is very important.
• The reader has to be able to connect to the hero. You have to feel what the hero feels to be drawn into the story. Like the first arc of the new Wolverine series. Wolvie is going out to "hunt" the people who killed that girl, and Rucka does an excellent job making us feel that Wolvie is doing what's right.
• Good pacing of story development. Whether it's just one issue or a multi-series arc, plot flow has to be appropriate.
• That leads me into the next point. Each issue has to have the abillity to stand alone yet still be part of the whole. If some random person picks up an issue of a series for the first time, they should be able to know what has gone on in the past and have some idea of what's happening now. Although I'd rather have an extra page of story, the first page recap in most Marvel series is great.