A few observations on Lurker (some of these have already been mentioned):
It does seem to be a slow card indeed, requiring a pair (or TR/KC) most times to gain actions (so not to risk opponents sniping actions you've trashed with their own Lurkers) but I can definitely see a variety of opportunities for it to be pretty strong and versatile in certain kingdoms.
It can gain potion cards without potions (e.g. Familiar, Possession), debt cards without taking debt (e.g. City Quarter), expensive cards like KC or Prince without the $, Grand Market with coppers in play, etc.
There are of course lots of cards with fun on-trash benefits from Dark Ages: trash a Cultist for a double Lab, trash a Squire for an Attack card, trash a Rats for a cantrip, trash a Hunting Grounds for a Duchy, etc.
Lurker can be a straight-up Fortress gainer--with Fortress in the supply, a Lurker in your hand can become a Fortress in your hand!
It's strong with TfB: keep your Apprentice engine going by continuously regaining trashed actions for fodder, pick up actions your opponent fed to Salvager to feed to your own Salvager, etc.
And as others have mentioned, it's great for pile control--it doesn't take too many throned Lurkers to drain that third pile for a surprise ending.
Lurker will be situational for sure, but it still looks like a lot of fun!