My vote goes toward Open as default when mounted. If I'm mounted chances are I'm on my cart, I open it far more than disembark it.
Also checking if an action can be done for the repeat button would be nice. It's not great to see R: Drink(repeat) on every tile after drinking once. Repeat seems to be a bit wonky on walls as well, you don't get a repeat action on most walls you continue, continuing a second story wall from the ground is a guaranteed way to run into the problem.
And last double clicking to target has caused more annoyance for me than not. If you have hellies attached to a cart or your bread any kind of agro. mob anytime you double click them to get info you target them. I slap my poor hellies with my axe enough without assistance. They rarely make it
Other then that I think it's fantastic, I've been here since beta and this GUI has damn near everything I've ever felt I wanted in the wurm GUI. Damn fine job.
Oh, an option to go back to the old font would be nice as well. I find it more legible, the smoothing on the current font makes me feel like I have to squint.