I'm trying to get CMinor down. It's really hard on my thumb because I have to press against the neck to get the pressure on the strings.
Of course, I'm learning it on a steel-string. I refuse to even try on my electric until I have it down on the acoustic.

I'm assuming you're talking about guitar, there.
The hardest thing I've had to learn (and still learn) is polyrhythms on drums. Try tapping triplet-quarter notes with your right foot, 8th with your left foot, and two other completely odd rhythms with each hand while retaining tempo. Then, try increasing the tempo of each limb...one at a time, then ritard back down. Practice this for freaking hours and hours a day...it becomes obsession. After 2.5 years of trying to perfect this....omg yeah

I love playing C#Major Scale on tenor sax.

Just thought I'd let you know.

That's because Tenor Sax makes everything sound sexy!

Practice polyrhythms on drums. Seriously, once you have that done...the rest of it just falls into place. If your goal is to just thrash and be able to play some fast fills...then no. But if you like soloing or playing just whatever it's totally worth the effort. Plus, that's the most impressive thing a drummer can do as far as anyone else is concerned. WOOO I'm crazy....

When I first started playing drums, I kept playing the simple beat that I'll show below. Just keep playing it over and over to get your coordination up, then start to experiement. I didn't play any thrash beats until I had some basic beats down pat.

top x's = hi hats
middle x's = snare
bottom x's = bass

x x x x x x x x x x x x x x x x x x x x
......x.........x.........x.........x..........x
x....x....x....x...x....x....x...x....x....x

Tried to line it up as neatly as possible. Just ignore the periods, it wouldn't space correctly without them. Consider them to be rests.

Um, I'm still way behind what I should be on guitar. That's of course a function of never practicing.

Oh! I know. They gave me a C Tuba in University Band this semester (bass open note is a C rather than Bb.) It's very strange.

Playing altissimo on sax has probably been the hardest thing on an instrument I've tried to accomplish, especially since I use a ridiculously strong reed.

A good non-plastic mouthpiece helps a ton, I've noticed. I had this same problem when learning to solo.

http://www.xs4all.nl/~marcz/Polyrhythm.html is a great example and defition of polyrhythms.

This one here is from that site. Incidently, it's the same one my drum instructor started me on. First, played one part with left hand and one with right. Then reverse hands. Then do the same with thing with your feet. Then interchange it anyway possible between limbs. After that, move on to more difficult ones. Once you start getting it down, start adding more limbs. VIOLA instant brain overload

THE KEY to this actually being able to help improve your ability is to be able to count it. If your limbs can do it, but only on autopilot, then it isn't working right. With most polyrhythms, you won't have to worry about "auto-pilot mode" being a hinderance...but with some more simple ones like this it may become one.

Yeah, perhaps. I made the switch from a plastic mouthpiece to a rubber one a few years back, and somewhere around that time I was finally able to hit altissimo.

I switched to a brass (I believe) jazz mouthpiece, increased my dynamics quite a bit.

Man, I don't think I'd ever be able to do poly rhythms. I think I'm rhyhmically challenged.

learning how to make lightning bolts fly from your guitar to smite the uncool... THAT is the ultimate challenge

I can shock myself sometimes. So I'm close to being able to do that.

Like this one?

nice ... hehe

Ugh, I forgot about that. I had to do a high F# for some audition that I didn't actually do, and even though it's the lowest altissimo note, I could only manage to get it out a couple times.

What on Earth is a double barre chord?

Most difficult thing for me so far is ... um. I can't say.

Like a Bb - 113331

I guess it's more like a 1.5 bar chord.

That's still a one bar chord to me, I play the 3's with my pinky, middle, and ring fingers because its easier and much more clean.

I'm pretty sure that's the way a guitar-instructor would tell you to play em, too. It really depends on the situation I guess, if you were switching from 113331 to xx111x quickly or something then I guess playing it with two fingers would let you switch faster...

How on earth could you play that with two fingers? If you held down the third fret, the e string should be on the third fret, not the first.

Not nessecarily, you can bar the 1s with your first finger, and the 3s with your 3rd finger. So if you arch the third finger you can avoid the e string easily. That takes awhile to get used to though.

Yep, I use 2 fingers for it too. It's not generally practical to have 3 fingers on the same fret at the same time on three sequential strings. Just makes things harder for me.

Learning tenor clef for bassoon. So hard, in fact, that I still cannot read tenor clef.

So let's say that my hardest hurdle was getting the upper octave even remotely okay.

I can't do it. Damn.

Well, I can do it, but it involves putting my hand in such a weird position that I can't bar with my other finger and wouldn't be able to switch to it quickly enough to play it effectively.

It's not because I have short fingers or anything either, I'm taller than the average guy and so I have longer fingers than average.