That's actually a really hard thing to answer, because we each have our own strengths and weaknesses. What's hard for me may be easy for you, or vice versa, and sometimes that doesn't even have much to do with our different levels of experience. There are certain aspects of this art that you might always struggle a little harder with, and others that might come more naturally to you.
Judging from the vast majority of demo reels and animation clips I see, however, I would say that people struggle most with the body mechanics overall. I see a lot of clips that have interesting acting choices, or cool eye animation or whatever, but none of it matters because their weight and balance are all off, or the anticipations aren't there, or the hips aren't leading the actions, or the shoulders don't move, or the overlap and follow-through is wrong, or (most commonly) the arcs are a jumbled and confused mess with sharp angles on the paths of action of the limbs, etc.
Any or all of these (and many others, of course) can contribute to the body mechanics feeling wrong, and this is the main reason I tend to recommend that people avoid dialogue shots until they have really spent some time honing their physical skills, and have created a few really solid physical action tests for their demo reel. I recommend focusing on the "ing" for a while. Anything ending in "ing." Get some shots of kicking, throwing, punching, falling, dying, lifting, climbing, jumping, etc. "Speaking" is the only "ing" I would avoid for a while, until your other "ings" are rock-solid.
THEN, and only then, are you ready to dive into the giant quagmire that is "Speaking."
Thanks for writing in, I hope that was helpful!