There is discussion about Duration Attacks in the mini set design contest, e.g. the one for
Seaside (LFN, if you read this, will that ever continue?
) but you'll have to wade through a bit to find the discussion points.
Taxation sounds alright, though the cost increasing part of it is extremely weak. It also functions as "discard 1 card", and I think that will have more of a negative impact than the cost increase. Often it'll just increase the cost of junk, which people won't be buying anyway. In fact, it is liable to help your opponents by buffing their TfB cards.
Usurper sounds a bit fiddly for real life play. Should I just flip my hand backwards? But then I'll have trouble seeing my own cards. Should I just lay them face up on the table? That could cause me to mix them up with cards that get played. It's also rather complicated for what should be a relatively weak attack. Seeing your hand as you play it usually won't matter
that much, and the penalty is far weaker than Militia in that the other player can run through his engine before discarding 2 green cards that he drew at the end, or just giving up an extra +Buy.
I suppose both of those are more just problems with specific implementations than the idea of having a "tag".
Great Embargo is not worded properly. You want to do it like Embargo:
Each other player takes an Embargo token.
When a player buys a Victory card, he gains a Curse per Embargo token he has. All Embargo tokens held by a player are discarded at the end of his turn.
The card does not need to be a Duration any more than Embargo does. It also seems weaker than Embargo. Early on, players aren't buying VP anyway. Later on, it is hit-or-miss whether you catch them on a turn they would buy VP. Even if you get them, it's just a Curse in the late game. New junk doesn't matter as much by that point.
Army is a great idea, but pedro is right that it doesn't need a Duration type. I'd generalize the wording too:
Each other player turns the top card of his deck face-up.
When a player would draw a face-up card, he turns it face-down instead.
You could do similar attacks, or even non-attacks. Each other player with 4 or more cards in hand puts one card face-up on top of his deck. Gain a card costing up to $6, putting it face-up on your deck. You could even remove the below-the-line explanation of what a face-up card means and just stick that in the rule book.
The big thing to consider is what it does to cards that reveal or look at the top of the deck. If you have a face-up Estate, does Rebuild hit it? If there is a face-up card on your deck, how does Lookout work?
It's a neat mechanism though, and naturally produces combo/counter potential with cards Wishing Well and Mystic. So that's cool.
Wine Cellar is problematic in games with more than 2 players in that it would be easy for other players to lose track of which cards are theirs, especially if you play multiple Wine Cellars. Having designated areas per player helps, but it would make the play area very messy.
Speaking of which, it is unclear how multiples stack. Suppose you play Wine Cellar twice and two of my Copper get set aside, one per attack. When you discard one Copper next turn, does this allow you to draw 2 cards? What if the third player also set aside a Copper -- one discard lets you draw 3 cards now?