I think the interface and implementation are great!
Have you trashed your big action card to a bishop yet?
Yep. It taught me to pay more attention to the game state and I haven't done it recently.
EDIT: That being said, I'm all for that sort of thing being more obvious (big red X or similar). The interface isn't perfect. It's just very good.
One reason I cannot get into Magic Online is that it's pretty easy to make mistakes that are absolutely impossible in the cardboard version, and you have to be paying attention fairly closely to make sure you play your effect at the right time as opposed to just saying aloud when you're playing something. Most people seem to accept it as a fact of life of Magic Online, and claim that they better understand the rules and how to time things now that they have to be careful. Me, I already knew all that and vastly prefer just having to say what I want and having it happen as intended.
But back to Dominion. If it's even remotely possible that you'd trash a card to Bishop that you intended to play, that is completely unacceptable. It's not like you're ever going to take a card out of your hand, put it on the table, and then have someone say "you never said you weren't trashing to my Bishop, therefore you must trash that card." Or similarly for discarding for a Militia. Those scenarios just never happen, because we have actual physical control over the cards and are able to declare affirmatively what we're doing and discuss with our opponents when we disagree about the game state. When you let a computer be the final arbiter of what goes, you need to make sure that the humans giving it instructions are actively aware of how their instructions will be interpreted; it is irresponsible to ever have the exact same human procedure for playing an action card and trashing a card without a significant change in display. Maybe if I ever get in the beta I can give a bit more pointed feedback on how to improve such things.
I played Magic Online a few years ago. After playing a few games, I actually felt it was pretty intutive, and I didn't make many mistakes. I actually thought it was fairly easy to use. It is a lot easier to make mistakes on Dominion Online which is weird considering Magic is a much more complex game. And, most people on the beta have offered good suggestions how to handle the whole Milita/Bishop scenarios such as having the cards flash red instead of green. I mean, why the heck does the card flash green when an opponent is playing a Militia? That doesn't make sense.