Worst is when people try to edit in max stats/items/whatever using a gameshark, end up making their file unplayable, and ask me to clean up after their mess. If you cheat by editing things you don't fully understand, you are 100% responsible for whatever you did to that file. Which means any resulting "bugs" aren't really bugs that i can fix; if you didn't back up your stuff beforehand, too bad, etc.
Now, if you want to try a mod considered "hard" by most such as Monster Tactics (which is the only significantly harder mod that I'd recommend), the main vanilla habits that need to be broken are approaching the game too much as if it's vanilla instead of as its own game. I've added several types of mechanics that didn't exist in vanilla, such as damage reduction, m-ev on monsters, r/s/m that don't suck, rebalanced formulas, etc; and make use of several underutilized mechanics like reflect and status effects in general. Note that this isn't to discourage playstyles, but to force the player to pay attention to the game's mechanics so that they can properly come up with a strategy for any given battle. Outside of that, go nuts!