I started to answer this question with a discussion of my own motivations for module development. They are many and varied, but they pretty much all come down to me either having an idea and some time or me getting paid to make time for an idea. However, I realize a person's motivations for writing his or her intangible "Drupal App" don't really matter if a particular motivation can co-exist with the existing model of community module development (whether it's evil or not, ignored or not, mirrored or not)... and ultimately I didn't want to do Robert's problem solving for him on what that could look like.
I then decided to take the angle of problems with the architecture of Drupal modules vs. an App store model, current support and maintenance channels, and a host of other problems I see with the idea of Drupal apps. Some points were established elsewhere... Yes, we can't really sell modules. Yes, people are selling themes. But no, I don't think selling "Features" would work... or at least I don't think it would be good for the consumer.
The traditional "App" (I hate that word - really? are we selling full applications?) must encapsulate its functionality, and it can't be effectively supported if its innards are free to be customized and mixed in with (i.e. broken by) other modules. This lack of interoperability between paid plugins on other platforms was actually quite a shock to me at last year's CMS Expo and is a big weakness. It also increases the development burden on the "App developer" making it a net loss for the consumer and provider. (Really, I saw an e-commerce plugin baking in its own Views system.) In fact, the only person I see getting a good deal is the "App distributor."
But I digress... these are just software / social engineering problems. If distributors really wanted developers and consumers to buy into their distribution models, they just have to provide solutions for these things. Once again, I'm not trying to do anyone's problem solving... unless they want to cut me in on their good deal.
What an App Store won't solve
I think other threads of this conversation have been dealt with adequately elsewhere, and overall I think an App store could work if all caveats are accounted for and the store's success is mitigated enough by the strengths of drupal.org to not blast holes in the community development model we currently have. One thing I don't see getting a lot of play in the discussion, though, is whose problems an App store wouldn't solve. I've been thinking about this since taking part in the discussion with ICanLocalize (sorry, can't find the link).
A paid distribution channel will not provide you with the time or money to code or support your module. You still have to make the initial investment in time or money to get your "App" written, and long before you're turning a profit you'll need to be proving your intent and ability to support your product. If you then rely solely on distribution fees to recover your investment, you're relying on the wroooong license to get 'er done. If you intend to bundle support and maintenance with the distribution, I just need to point out again... this is first and foremost a good deal for the distributor unless he turns your App into Angry Birds.
Really, though, this is freaking Drupal... all we need is for enough interested people to build an installation profile for paid module support. Does that mean every module's support would go behind a paywall? I sincerely doubt it, as again, developers still have to prove their competence to support their project to non-paying new customers to convert them in the first place. They're also not the only ones who support modules on d.o... every one of their users is a potential support provider.
I do think that publicizing intentionally scaled back support in the absence of a paid distribution channel is not a good way to win hearts and minds. You don't have to say it, just do what you need to make money and offer premium support for people willing to pay for it. There are also other ways to build support around your module in the community, whether it's a bug squad or a sponsored development sprint.
Leaving module developers behind
Finally, something that just hit me... the only "Apps" I see as being viable at present are SaaS plugins and themes (thanks to licensing opportunities). Others see distributions and Features-esque configurations as viable. Perhaps there are still other viable options, but at the end of the day, none of this translates into dollars to fund the module development that any of these products depends on. And isn't that what started the conversation in the first place? Are we trying to figure out how to support module developers, or are we looking for a different way to make money using Drupal + contributed modules beyond professional services?