Hmm...it's clearly best to avoid arbitrary inconsistencies, and I think it would be great to have a set of conventions, design guidelines and best practices. But it seems to me that having an ironclad 1:1 correspondence between specific keys and specific types of actions (opening a tab/window, inserting in the the current doc, transforming the current selection) is not necessarily the answer.
Specifically, it seems most important to me that <return>/<enter> consistently mean "execute this command." And since some commands are inherently "go-somewhere" types of commands and others are "insert-something" types of commands and others are "modify-this" types of commands, that would imply different results when hitting <enter> for different commands.
Such a system does make each command responsible for setting user expectations, via an adequately descriptive name (e.g., "go to yelp" vs. just "yelp") and an informative preview, but I think that's a good thing.
For cases where the primary function of the command is to display something in the Ubiquity preview pane, or where a secondary function exists (e.g., insert the Yelp summary in the current page instead of going to the full Yelp page), then I agree that adopting and promoting some conventions (both for specific keys and for visual presentation/selection of available functions) would be very helpful.
- Gray