There's another possibility, though, and I'm surprised that no-one seems to have picked up on it thus far. Google's Google Apps thing provides private email, calendar, and so forth, to various corporate users and universities. Wouldn't it be nice if a small business could simply go to a website and buy themselves, say, a CRM, or an accounting system, or whatever, maybe paying by user count or per year, which would be private to them, hosted by Google, and integrate nicely with their existing Google login system? I suspect that Google will do something like this, something along the lines of Apple's proposed iPhone software store. The developer will get a cut, Google will get a cut, the users will have their application without having to worry about hosting it themselves... Everyone's happy. Except the webhosts, and the traditional vendors of custom corporate software, but it's not Google's job to look after them.
Also this, which is sort of obvious, but took over a year to show up:
Some form of cron-job-like functionality; that is, the option to have a function executed at set intervals. This is essential for all sorts of applications, and could be relatively easily implemented in the current model.
Anyway, today they launched a thing to allow the integration of any web application into a business's Google Apps for Domains, and the selling of access to such applications in a marketplace, so I suppose I was sort of right.
This 'app store' meme seems to be taking off very quickly...