Ed here: Most of us mid-listers look for the break, large or even middling, that will bring us a substantial number of new readers. Not necessarily the Big Break itself but the a slot near the top of the genre bestseller list. There are number of good sites that offer excellent advice on the subject of writing and promotion, everybody from M.J. Rose to Doug Clegg to Holly Lisle and Paperback Writer (which you can link to from my site).
Don D'Ammassa http://www.dondammassa.com/index.htm is both a very good writer of fiction and an excellent reviewer of it. His books can be found on shelves as various as mystery, science fiction and fantasy. In one of his recent reviews of the latter, he opened with a paragraph that I think speaks to fiction writers of all kinds in this cluttered and sometimes dismaying marketplace. I think he speaks to most of us.
In most cases, the difference between a "ho-hum" and an "ah-hah!" in fantasy fiction boils down to whether or not the author has found a new schtick. The vast majority of fantasy novels - and the same holds true of other genres as well - are reasonably well written, competently plotted, and potentially entertaining. But if you read a lot in any one field, there is a tendency for that big glut of books in the middle of the spectrum to get blurred together, and if you read enough of them, they start to feel stale and unsatisfying. That's not really the fault of the authors, and since the turnover of readers is presumably self sustaining, there will always be some component of the audience that won't be as jaded. On the other hand, for those of us who read quite a bit, what really sticks in our memories alongside the really great few are those from that fall just behind, but which exhibit something new or interesting.
Ed here: I think this raises a question most of us need to ask ourselves each time we're planning a new novel. Even raising the issue will likely make some writers uncomfortable but whether we like it or not editors in houses large or small are certainly raising it when they look at each submitted mansuscript.