I mentioned this in my Superman rant, but I think it's worth expanding, because I think it's a major problem with mainstream comics right now. The bottom line is that there is a big difference between a serialized story and a story released in installments.
Starting with some books in the Silver Age, serialized storytelling became the norm for comics for decades. The ultimate goal of a serial story is to keep the reader coming back, month after month. This is accomplished through building up multi-part stories and developing continuing subplots that link the major stories together. The pinnacle of this is the Levitz Paradigm, Paul Levitz's moderately famous method of developing subplots over time and promoting them to main plots to maintain forward motion. At its simplest, this is what serial comic strips do: one panel recaps previous events, one panel introduces new action, and one panel leads into the next installment. The idea in serial storytelling is to generally maintain a flow from one story into the next. The advantage is that it gives momentum to the series and makes each new story feel like it grows naturally out of the last one. The disadvantage is that it relies on and requires some sense of continuity, which can make writers feel hampered and could potentially scare away new readers1. Such comics are also difficult to collect in trades
For the most part, this wasn't the case throughout the Golden Age and well into the Silver Age. Comics had very little continuity, in part owing to the demands of the distribution model--comics were distributed at newsstands and grocery store spinner racks, and there was no guarantee which titles would be available from month to month, making it difficult for most readers to follow any one series regularly. Instead, comics were released serially, but each with a mostly self-contained story (so to use the terminology above, each story was released in a single installment). The advantage of this model was that anyone could jump on anywhere; the disadvantages were that anyone could also jump off anywhere, writers were hampered by the inability to tell longer stories, and any continuity accumulated largely unintentionally. It was the growth of the direct market, a major change in the distribution model, which allowed the serialized story model to take over.
The installment story didn't die out during the reign of the serialized story. Instead, the direct market allowed for it to flourish in new forms, no longer tied to ongoing titles and anthology books. Direct market distribution meant that readers could be reasonably expected to follow multi-part stories in a way that they couldn't in the past, and the growth of comic shops meant that interested customers were being regularly exposed to a lot more product than they'd usually find at a newsstand. All this led to the development of things like the mini-series, the maxi-series, one-shots, prestige format books, and original graphic novels. But it seemed, in many cases, that the hope of many of these installment stories was to test the waters for or lead into the real moneymaker, an ongoing series (generally told in serialized story format).
Today, the model is shifting that again. The direct market has become the main avenue for comic distribution, and the casual market has died out almost entirely. But now, the growth of the trade paperback has allowed chain bookstores and Internet shops to pick up a huge portion of overall comic sales. This change in distribution has allowed for another shift in comic storytelling, away from the serialized story and back toward the installment story. The mini- and maxi-series model of finite, largely self-contained stories has become the storytelling model for most ongoing titles, replacing the serialized story model. Again, it's looking at the big moneymaker, which is no longer the ongoing series but the collected edition. In part, this is because bookstore and Internet distribution bring up a lot of the same problems that existed in the days of the newsstand: there's no guarantee which volumes will be available at your local shop, and stand-alone self-contained stories are easier to justify purchasing (while numbered volumes may be daunting to casual readers). Trades rely at least as much on character and creator popularity for sales as they do on continuing stories.
Right now, the individual periodical comic (or floppy) is having a hard time justifying its existence. It seems to exist for three reasons: to maintain the market provided by comic stores, to ensure that comic creators get paid regularly, and to produce content to be collected into trade paperbacks. That's pretty problematic; trades are largely ad-free, more durable, take up less space, contain complete stories, are often easier to obtain (Amazon and Borders deliver to my house), and are often cheaper than their floppy counterparts (especially since Amazon and Borders offer better discounts than my local comic shop). Right now, the best reason to keep buying floppies is to stay on top of the stories as they happen, and that's a pretty flimsy reason.
I think DC and Marvel are looking to electronic distribution to be the new justification for floppies, but I don't know how well that will work unless they cut way back on print runs to account for the people who'd rather just fill their hard drives. Still, I think people (especially comic fans, and especially people without iPads) generally like to have something physical (which is why CDs still exist), and as long as periodical comics are focused on writing for the trade, they're going to see readers waiting for the trade. In order to justify periodical comics, I think the pendulum needs to swing a bit back toward serialized stories. Hopefully, digital distribution will help make that a possibility, but I'm not terribly optimistic. Right now, most ongoing comics seem to be mini- and maxi-series loosely tied together, if at all.
Or comics need to find a new balance, using building subplots to link together installment stories. There have been books which use variations on that model, like Blue Beetle and Incredible Hercules, but they've become the minority rather than the majority. I think there's room for both installment stories and serialized stories in comics, but I'd like to see a little more effort put into giving them the room to do what they're best suited to. Let OGNs be OGNs, don't dismember them into floppies. Let ongoing series be ongoing series, not strung-together miniseries. And let's realize that you can't float the entire industry on the Wednesday regulars--especially if your target demographic is the trade-waiters..