For directions, the key parameter determining which directions mode is selected is
dirflg. (As of May 2013.)
- Driving: [none]
You can append any of these to a Chrome search engine string, so a Google Maps search that defaults to transit directions would look like this:
If you want to also show the transit lines overlay, add in
You can control whether satellite or map tiles are shown using
t=h for satellite or
t=m for map. This setting appears to be sticky from one Maps session to the next, though, so it's less necessary to include in the search query.
These queries work with both individual addresses and directions (e.g. "Times Square to McCarren Park"). They don't include a default starting address as Benj suggested; I do like that idea but I prefer this greater flexibility.