I think that all books and all artists are different, and while we’d all love it if everybody could produce as much work as steadily as guys like Mark Bagley, John Romita Jr and Salvador Larroca (all of whom, I think you’d have to agree, are clearly the ‘lead artists’ on their respective titles) that simply isn’t the case in most instances. But in many cases, the very work that makes the artist so slow is the reason why they might be popular–so you can’t get more issues out of them and expect the same result. Hopefully, though, you can find artists who are at least within the same stylistic ballpark with one another so that a given title has its own specific aesthetic.
I don’t know that there’s any one thing that books sell on any more, rather it seems to be a combination of things. That said, there are certainly artists who command a following, and who have fans that will support them and follow them from project to project. That said, I can’t think of the last time a new artist hit the scene and so galvanized the readership that he became an enormous sales draw in and of himself. The guys who still demand that sort of draw power tend to be of the last generation at least–your Jim Lees and Joe Qs. Who was the last new hot artist that we saw?