I'm new (and bad) at this game:
-STREAMS. Can't do them at all. Especially high bpm streams, long streams, highly spaced streams, streams that lead into a slider (especially if it's a fast moving slider), streams immediately following a slider, streams infused with both regular hit circles and mini sliders. I also have a problem with my mouse accuracy on really long streams, I get confused with the mess of seeing a gazillion approach circles and scorebursts that I can't even tell where my mouse is supposed to be anymore and then my streams end up looking like 300-300-300-100-100-50-x-50-x-x-x-x...x-x-x-fail
-Spinners. I use touchscreen + keyboard usually and I either barely pass spinners or miss them completely. I'm a million times better at spinners with my mouse, but I can only play up to normal/low-end-hards on my mouse. Sometimes I'll literally pause the game, grab my mouse, unpause, do the spinner with my mouse until it's completed, re-pause, get adjusted to touchscreen, re-unpause; however that only really works on long spinners. I'm also bad at short spinners in general, by the time I comprehend they are there, I miss them. I also hate single notes immediately following the spinner, especially when the single note is in the middle of the play area because then I'll usually end up confusing the spinner's approach circle with the hitcircle's approach circle.
-Really high AR and really low AR.
-Quick patterned jumps; I tend to hit the circles in the wrong order. ;~; And jump patterns that stack on top of themsevles. Other than that I'm fine with most jumps.
-Flashlight trololol (I don't have the patience/skill to memorize a map.)
-Really small hitcircles...if I see a map with those, it's auto-(pause->Back to Menu).
-Getting a 100 in the last few notes of the song when I had the SS the entire time, getting a miss in the last few notes of the song when I had the FC the entire time, failing in the last few notes when I was passing the entire time.
-Really fast sliders, especially if the slider is some weird shape.
EDIT:
Taiko:
-STREAMS
-accuracy
-I either hit every slider perfectly or miss every single tick in the slider entirely.
CtB:
-STREAMS (especially really dense stream)
-triplets
-trills
-typewriter-esque patterns like:
.... /
.../..
./....
.... /
.../..
./....
.... /
.../..
./....