This is a broad subject, but definitely an important one. Probably every card has tactical decisions associated it. Here are a couple off of the top my head. I think the concept of tactics is maybe broader than this, but I'm interpreting the question as
What decisions are important with respect to playing the cards that you already have, as opposed to ones that you will get
For me, the classic card is Wishing Well. This is like a complicated optimal stopping problem. Someone can probably write a lot about it, but this is the tip of the iceberg for me.
1. Optimal Play Order of Wishing Well You have Wishing Well and other cantrip type-cards. Do you play the wishing wells first, last, in the middle? You could play them earlyish, and wish for your most common card, i.e. copper. But if you have other card drawers, as you draw through your deck, your posterior distribution for the remaining cards gets lower in variance, and in the extreme case, you always know your last card. Or do you play it last, wishing for a particularly important card? Your expected +card goes down, but that's not the whole story.
2. In general, play cards that require decisions last This is a trivial observation, but players mess it up for no reason.
3. Trigger a reshuffle The first question is whether a reshuffle is good or bad for you. If you have a lot of bad cards in hand, it's good. If you've played through an engine and discarded a lot of bad stuff, it is very very bad. In more intermediate cases where a shuffle is more moderately bad, consider the tradeoff. I'd generally play a Witch even if it triggers a reshuffle.
4.Masquerade + Discard If your opponent makes you discard to three cards and there's masquerade, save a bad card.
5. What to discard In a game with torturer, militia, etc, never (ok usually don't) discard your engine pieces if you're building an engine. Always discard your treasure first.
6. When to play gainers This maybe violates my definition. If you have a strong engine, you should realize the opportunity to gain cards for play the same turn. Picking up another torturer via upgrade and playing it that turn can change the course of a game. So don't just save them until the end after you've played three labs for zero cards.
More if I think of them. Great topic btw.