My experience with steam is that for a good experience, the game must work as a whole out of the box with no problems, issues, missing help files etc. many players are lazy players and you also want to make a first good impression.
I do think that creating an up to date manual may be required before the game goes on Steam... (however the Greenlight process can be started while the Manual is worked on).
I think there was once some issues with the Artistic License and Steam. Or more precisely, a closer intergration with the Steamworks API is not compatible with the GPL. Not sure about the Artistic License though.
I have checked through both the GPL and Artistic licence, and neither present problems with putting the game on Steam, although the Steamworks API is another matter. Under GPL, the Steamworks API can be used so long as it can be easily removed without cripling the game (i.e. it is not an integral part). For the Artistic License I am still trying to determine if there are any issues with integrating Steamworks.
Steam may have the big advantage of solving the SDL installation issue on mac OS, as well as the eternal 32 bit/64 bit Linux issue. That comes with a price. There are so many pak sets: Which should go with it to not overwhelm a casual user? (My impression is that any pak which forbids bridge/tunnel for single height steps will frustate a novice.)
I would presume that the default Pak would be used and the others would be available as DLC or through the Steam Workshop if that will work for the game. As far as Mac installation goes, someone with a Mac would need to provide the Mac package for Steam (as packaging for Mac requires using tools that only work on a Mac, to capture file execute/run permissions etc). I would have no problems doing the packaging for Linux and Windows.
Also in regards to the Pak's, I think giving more descriptive names when released on Steam will make it easier on Steam users (i.e. if the 'Pak' one is renamed Default (or something similar) for the Steam Release, so when it displays the list of Available Pak's users have more of an idea of what they are looking at.)
Afaik, steam always downloads the game when "bought".
Steam downloads the game when the user clicks install, and keeps the game on the computer until / unless the user tells Steam to remove the files. When a new version comes out, it then patches the files using a diff method, unless the user has opted not to have automatic updates.
As far as the 'requirements' for listing the game on Greenlight (the first step to getting it on Steam), the only requirements are 4 pictures of the game, and a Game Play Video hosted on Youtube (preferably without Adverts enabled, otherwise the ad's will come through to Steam, which is not a good look when trying to get a game through Greenlight).
Mention of the Simutrans Experimental got me thinking:
Steam allows users to opt into various 'beta branches' of the application.
If Simutrans is put on Steam, would it be desirable to add the Simutrans Experimental fork in as a branch on Steam?
Or would it make more sense to focus only on the core Simutrans build?
Also, would it make sense to add in a 'dev brach' based on the nightly builds, but not necessarily updated nightly (if it could be integrated into the nightly build process, then that would be fine, but otherwise, do a weekly or monthly update of the dev branch depending on how much work is involved).