Whatever do you mean?
:;Proceeds to tell the DM that he'd like to roll Perception on the door ahead and then spends an hour of real time deciding how to go through it::
There are several types.
Some are mechanical. In D&D, we get used to "your turn = move + action + bonus action" in any order, and move can be split. In other games, your move
has to happen before your action, and once you've stopped moving, you won't be able to move until your next turn. Other games don't discriminate between moves and actions, or else allow two minor actions instead of a single complex action, etc. This usually take 2 minutes to learn, but several games to become efficient with the new type of action economy.
Some are about problem solving. In D&D, spells are readily available and magical/supernatural abilities easily solve many problems to the point that they basically become non-issues. For example, in other games, the ability to produce light and/or ignore darkness is both rare and significant, and it can take a while to grasp the implications of that.
Some are about lethality. D&D is generously heroic and players are encouraged to take life-threatening risks on a daily basis. Ways to avoid combat are rarely explored or even considered boring. In other games, this reckless approach will get you dead, give you many enemies, and force the DM's to decide between TPK and bending the game and setting's expectations.
And some are about genre expectation. D&D is very combat-oriented. Characters are capable and can grow into realm-shaking goddamn superheroes. Fragility is not a big theme in D&D, but different games with different genres adopting different themes take some used to.
"Things to unlearn" are not meant as bad habits; they are associations. These are normal and to be expected (and to a certain extend, desired), but they can conflict when the game changes. And that often takes more time than learning the actual rules of play.