What sort of user interfaces the web is good for
Here is a corollary to what standard interfaces are good for, in the form of a thesis:
The web excels at standardized interfaces, but requires increasingly heroic amounts of work for excellent customized ones (once you want to step out of what the browser gives you). This implies that web applications are a natural fit for occasional usage things, where any awkwardness of the web interface is overcome by the advantage of not having to learn yet another custom interface that you use for ten minutes once every month, or for things where the natural web interface is a great fit for the task, but they are not a good fit for anything else.
(For example, Google Maps is very nice, but in a sense it is a bear dancing. As Google Earth demonstrates.)
This raises the obvious question: what things are a natural fit for the normal web interface? I think that the answer is filling in not too large forms, or at least forms with relatively little interaction with the system (for, eg, immediate error checking on some fields), and navigating through information in relatively simple ways that have only a few choices. Usefully, I think it turns out that there are a lot of applications that don't want to do much more than that.
(This suggests that one reason that people actively like web applications over their old non-web versions is that they were sick and tired of programmers coming up with yet more interfaces for what was fundamentally filling in forms and doing basic information navigation. Possibly this is an obvious corollary.)