For me, it is planning - though I have to admit that I usually stay with shorter stories or "only" scenes if I get to writing . I usually have an idea and just want to "jump into it" - and doing too much planning just gets me out of that moment... maybe that will come one day . Though for planning, I see two things: "detailed planning" of the story arc, but somehow also everything around (would more get into world developement).
What I also really don't like is naming characters... in all the stories or anything I developed so far, I never really had a thing for names. There are very few names that stuck to a character, and with the rest it was just like "Hm, obviously needs a name... why not XY, and maybe change it lateron".
Good to know I'm not the only one! I recently saw an interview of an author, and he said that he always needs to give the people names before he can write with/about them, but an author he visited named her characters A1, A2, ..., and assigned names afterwards.