My worst habit is definitely that I'll get into a character, and because the game is new, there's a certain magic and immersion to that character. And I'll usually get like a halfway or 2/3 of the way through the game, then I get interested in the build system, the meta, etc., and start a few new characters, to try things out, and stop playing that "main."
But the problem is by that time, although theoretically now that I know the systems I should be able to start a new character knowing what I'm doing, with full immersion, I've done the early levels so many time that I'm bored to tears with them.
Then I think, well, I'll go back to the original character, but by that time the magic has worn off of course, and meanwhile I'm annoyed by all the poor decisions I made with the character while I didn't know the system - that old "main" now seems botched beyond repair.
Also, I have a horrible tendency to waste tons of time arranging my inventory and UI - like, I'll get it just right on one character, and so then I have to do it the same for all my characters.