Regarding keyboard: The idea of the Premium (aka Steam) version of DF, was to make it more inclusive for new players. There have been requests for a "mouse only" UX. Toady and Three Toe jumped to the conclusion that the original keyboard bindings were too complicated and undesirable. So Toady redesigned the way the UX worked and replaced all of the keybindings to work well with the mouse (chief amongst it is to have left hand centric movement keybindings with ASDF and to use the mouse for basically everything else). At the same time he removed all the old keybindings and made it so basically they don't make sense any more (because the menu breakdown and ordering is different).
When it released, quite a few older players were disappointed that the old keybindings didn't work. Particularly bad is that you can no longer play without a mouse. Apparently this was an unexpected complaint. Some older players basically said that they couldn't play the new version at all.
Tarn has said that he will slowly add the ability to play the game keyboard only. However, the old keybindings will forever be impossible because the UX has changed so drastically. He's not open to adding a special UX for older players (which I think is completely reasonable). It's been a bit of an unfortunate thing. As an older player who liked the old keybindings, I will say that the new keybindings are subjectively worse IMHO. The general playability is better, though. I've completely rebound the keys for myself and I no longer even remember what the default keybindings are. I think once full keyboard ability returns, it will be just as good, but it will take time (and I'm still pretty negative about the current defaults, but it's very easy to change).
In terms of Linux support, the latest beta version on Steam has native Linux support and it works beautifully. It should ship very, very soon. My only complaint is that text mode (i.e. in a terminal) is not currently possible and probably won't come back for a very long time (if ever -- Putnam mentioned that they thought it might be quite difficult).
Again, as an older player, I think the current version is a massive upgrade overall, but there are definite pain points. I feel sympathy for those who find the new UX unplayable, but it is definitely a very minority opinion.