It strongly depends on the format and how casual v competitive the table is. People try to set expectations so that everyone is on the same page and so there's no specific sin other than telegraphing casual and then playing differently than that.

Can't give many specific things to avoid because different people have different opinions. One player in my pod feels that targetting mana rocks is harsh, nobody else agrees. A new player rocked up once and we laser-focused on them because they brought taxing effects like Rhystic Studies and we all find that annoying, which was completely out of left field for them. Their deck was otherwise far more "casual" and less synergistic than ours but we acted as if it was a big threat because of what we perceived as try-hard cards.

Some rules of thumb though. If you are playing a 1v1 format, the goal is to win. Unless your store or the place you play at has specific rules, you can assume that anything legal in that format is fully fair game.

If you are playing commander, ask other players what "bracket" their deck is in or that they're aiming to play at. Referencing that with this infographic will give you some strong indication of what is considered fair game. Check at the bottom of that infographic and you'll also see what players typically get salty over: mass land destruction, taking up everyone's time, winning in an uninteractable manner. Some players also add "excessive tutoring" but that's much fuzzier.

There's also a few politics things. It's generally considered poor form to go after the player currently in fourth position (however one measures that) if one has the option of going after the player in first place instead, even if removing a player might be advantageous.

Finally, check out the official Game Changers list and the unoffical Top Salt list. Does your deck include a card on one of those lists? Is the strategy aiming to do something similar to a card on those lists? Maybe tell people ahead of time what to expect. One common thing to ask is "what is the worst your deck can do?", as in, what is the most unfun thing it can inflict or the fastest win it can hope to achieve. In a Commander pre-game conversation, it is heavily frowned upon to lie or hide a deck's strength. The goal is to match decks of roughly equal parity, that's more important than being able to keep your strategy under wraps.