I mostly agree with Attentiondeficitdisorderly Too Flat about manga in the American market; sure, I have a couple of quibbles (for one thing, I think that the "unflipped" format was a gamble that Tokyo Pop took based on cost, not to enhance its coolness) but on the whole I agree with Sean Collins's prescriptions for American publishers except for this:
1) Stop writing continuity-heavy or continuity-dependent stories that require a familiarity with the characters unattainable to the casual reader
Most of the really successful manga, both in Japan and in the US is completely continuity driven; the stories may move in arcs, but the series as a whole is one long story with a beginning, middle, and end. Even Dragonball Z and Ranma 1/2, which are much more episodic than usual, do actually wrap things up (more or less) at the end. The others that have gained serious traction among the new readers, particularly the currently top-of-the-heap Chobits and Love Hina, are determinedly serial. The two real differences that I can discern between them and American "mainstream" continuity-driven comics is that a) the paperback format encourages stores to keep them all available back to the beginning, and b) the continuity is strictly within a particular series, no cross-overs with other titles; cameos, yes, and as with many generalizations about manga, CLAMP is an exception with its CLAMP School Universe (although even there, the story doesn't jump from book to book--reading the other books in the setting just lets you know more about certain characters's back-story). Of the two, I'd guess that a) is far more important. You can jump on any time, not because the story works in complete recaps every few chapters, or gets rebooted, but because you can just buy #1 any place that you can buy the most recent.