I think it would be from a usability POV very nice to have the following possibility:
1) Suppose the user selects two changesets in the "History" view
2) User toggles view mode to "Diff" view
3) "Diff" view automagically preselects the two changesets from "History" view and shows their diffs right away
Furthermore if the user selected only ONE changeset in "History" view MacHg could offer that as the changeset to compare against right from the start.
How does that sound?
//If the user, however, selected more than one changeset, I guess one would have to find a way to deal with it in a deterministic manner, although I don't really understand now why one actually can select more than two changesets in "History" view anyways... But perhaps it has to do with patches, queues or undos and stuff!?//