How often do you play Dominion 'for fun' nowadays, or does it feel mostly like work? How competitive are you in those games?
It's almost always playtesting. There's enough playtesting to get in all the Dominion I want. Sometimes there's an event of some sort where I play some games for fun, but like, that's one or two evenings a year.
If you mean, how much do I win, I win my share. If you mean, how much do I try to win, I am mostly trying to win, but when there's a new card to playtest I will often feel obligated to buy it, see what it does.
In general, are there any sorts of abilities (Villages, +Buys, etc) that you would never play without (in a non-playtesting game)?
No, I'm happy to see random cards come up and have no villages or whatever this game. If I thought it was best to have a village in every game, that would be the rule.
In my group everyone's far too polite and no-one wants to be the first to buy an Attack, while we love the mutually-beneficial player interaction cards. How important do you feel Attacks are to the Dominion experience? Do you prefer to have zero, one or more Attacks in a kingdom and do you like to ensure there's a Reaction when you have them?
There are players that specifically like attacks, but for me interaction is what's important, not attacks; later Dominion expansions have fewer attacks but don't go low on interaction.
I don't ensure there's a Moat or whatever available. For playtesting I may want to test whatever specific interaction, and that could include testing an attack with Moat, but the intention is that usually there's no Moat, and then that sometimes there is one.
Last boring question, even more optional than the rest:
Do you play with any sort of 'if someone didn't notice when I played Council Room and it's their turn, they've missed their chance to draw a card' rule? Do you often have experiences with people forgetting to do things (I find it happens a bit with Amulet) and how do you deal with it?
For us it's the opposite. I want a good clean fight, with the rules followed; if you play Council Room and don't make sure everyone knows, I will be all over that, dude, you have to make sure we know about your Council Room.
When people forget something a lot, I look into fixing that, changing the card, getting rid of that issue. I am okay with the specific demands of Amulet though; we all remember Amulet.