Haven't we all encountered a piece which we love dearly and where everything is within one's ability, EXCEPT for a single passage (even after practicing it for long), and there's no way of faking said passage, so we decide to give up all hope of playing the piece? So, why don't we confess these pieces/passages? And just maybe others can give advice on cracking these passages (but this is just an added benefit, not the primary reason for this thread). I'll start:

Brahms: Piano Sonata No. 1 -- I can't manage (in tempo) those skips in the last movement's principal theme (and it reoccurs a lot!).

Brahms: Piano Sonata No. 3 -- The last movement's reoccurring sixteenth note passage that first appear in bar 25, and especially its reappearance in thirds later on -- jeez, how does one play those in tempo? This is especially frustrating because the rest of the sonata is within my grasp, and I even played the first movement successfully for a competition during my teen years!

In my view there is no such thing as "can't
be played." I believe that anyone, even
an amateur with no talent, can play anything,
given enough time and effort. You say:
"even after practicing if for long." The
key here is your interpretation of "long."
Apparently, you follow the conventional
interpretation that if one can't
work something up in a few months, then
it is too difficult and you should drop it.
Practicing those difficult sections
for several months is not "long" in my
view. "Long" in my way of thinking can
be like 5 years of more. So the reason
you can't play these difficult sections
is simply because you haven't worked
on them "long" enough. If you want play
those difficult sections, then start
working on them and keep working
on them until they're playable, which
may take years, not mere months.

Sometimes we have to take a break from such pieces and just come back to them 6 months or a year later. You'd be amazed at how much easier those passages get! Usually, it's not that we can't play it, it's just we aren't practicing in a manner that will allow us to play it.

Having said that, I know there are pieces that I'm simply not ready to tackle yet, like the 3rd movement of the Waldstein sonata and those darn trills! There's Mozart's K.284 at the beginning of the development section that is a bugger to play, but eventually I was able to figure it out after making adjustments to fingering at least twice!

But I think revisiting these pieces later on will really be quite different as I gradually improve technically.

Double thrills in one hand make a piece too troublesome for me to study: therefore I'd play Chopin's Barcarolle and Beethoven Sonata opus 111, if they didn't contain the double trills.

Why not make them single trills then? If that was the only reason preventing me from playing a certain piece, I'd simply rearrange the difficult section to better suit my needs. I'd bet that 90% of the population wouldn't even know the difference.

Brahms 2: 1st movement- LH passage
2nd- Impossible Octaves
4th- Impossible Thirds

Granted, I doubt I'd be able to pull off the rest of the concerto within even the next 10 years, but it's a very long-term goal of mine.

dnephi, which LH passage in the first movement are you talking about? Can you provide a bar number or describe the passage and/or location? And I agree that those octaves and thirds are killers!

Mm. 146-155, Approximately.