It seems like an especially hard question to answer – especially without using subjective terms.
For example I would say that the following are all trivial:
- Use of unobtrusive scripting to layer on basic page behaviors (ex. Twitter).
Whereas the following are sufficiently complex as to be deemed not trivial:
- Applications that do Ajax requests to a server-side component (ex. GMail).
- Applications where the majority of the initial download is executable code (ex. 280 Slides).