Thoughts before reading 160+ replies in this thread...
Debt is a surprise to me. Similar ideas have been discussed on fan cards. I like that it's present on multiple cards as a set mechanic rather than on just one specific card, justifying the extra complexity and components. But it's also actually very simple. Instead of having to resolve interest, it's just an easily understood ban on buying anything else.
It'll be interesting to see how Debt-costing cards compare to other cards with non-Debt costs. Potion is still pretty easy to grok, in that the Potion-cost limits you to gaining only one Potion-cost card per shuffle per Potion you have. The need to line up Potion with other money means that more expensive Potion cards like Golem and Possession are that much harder to get.
OTOH, Debt-cost cards are easy to get, because you could even get them for free. But you'll pay for it later, and that can be a real problem if you're not careful. Neither City Quarter nor Royal Blacksmith seem like they are worth $8 up-front, but splitting the costs into multiple turns makes them more palatable. In a sense, it's like Tactician where you sacrifice one turn to make another turn way better. In this case, you get the good turn first (where you buy the powerful card) and then weaken your next turn or two while you pay off the debt. Capital emphasizes this multi-turn pacing even more by advancing you $6 that will be taken from your next turn(s).
Theme-wise, I really like Royal Blacksmith. He's a particularly skilled Smithy, but he refuses to work with inferior metals. Nice.