I think I really like this. My biggest concern is that it does too many different things. The main idea for the attack is pretty nice, and I think it could just be combined with a simple vanilla effect (maybe just a cantrip). As it is it's really cool too though. Playing the top-decked action card is a fun and unique effect, but it might be really crazy in some situations. In thin decks where you expect them to have all actions anyway, moving it to the top usually won't be helpful for them.
Here's a weird edge case I just thought of. If they have only one card in their deck, and it's a Mysterious Door, you have to discard it, because if you don't, you get stuck playing it over and over until you do. I think this is okay in 2-player (equivalent to revealing a Moat forever), but in multi-player it can get weird. Suppose one player has a Mysterious Door and nothing else in his deck, and another player has a Woodcutter and nothing else in his deck. Now you can generate unbounded coin and buys, by just having them both top-deck it over and over. It's also hard to track, since they're all nested. I'm actually not 100% sure it happens like that, because the Woodcutter/Mysterious Door might be stuck in being revealed land until you finish resolving the revealed Mysterious Door. I'm getting really confused now...