I want to play a piece X, for sake of argument, let's use "Fantaisie Impromptu" by Chopin. How do I decide whether this is a good piece to play to expand my repertoire next?
I am looking for factors to consider, such as:
Edit: I know this seems stupid, but can the above example be used as well please?
I judge a piece's difficulty by how easily I can sight-read it. If I can sight read it quickly, it is too easy. I might polish it and perform it in a low-key setting but probably not in a serious performance.
If I can sight-read the piece playing very slowly, this is "just right" for the level of difficulty. I am not suggesting that everything would be correct going slow, but that I am grasping it in general.
If I am playing very slowly and still getting extremely confused, or if I am overwhelmed just looking at the music, I will put that piece aside for a later time. I would have to really love the piece to be willing put in the immense effort to learn it before I'm ready.
This is the way I do things, but I am a strong reader. I know of some technically fine pianists who are not strong readers, but I can't say how they might judge the difficulty of a piece.
Age doesn't matter. I think if you really love the piece, you will be able to pull the musicality out of it. Listening to recordings helps with this, too.
Thought-provoking question. Obvious reasoning - if you cannot play up to its technical level, it'll be a long time coming. If you like it, and see yourself playing it - in a short or long time, yes do it. It may be a long, difficult piece that goes on the back burner, and takes a couple of years to perfect. So what?
Experience. Doesn't mean a lot. Some people achieve in six years what others achieve in six months. Age - it's a number! If you think a piece may be more suitable for kids, learn it and play it for kids! If it's a serious classical piece, learn it for the sake of playing that sort of music - and perform it for an appropriate audience.
On that tack, it's worth being able to play different styles of music. Some places I play, some of the styles I play would go down like the proverbial lead balloon. Others - that's all that works.
I expect there will be any number of answers that focus on the technical skillset required to play a piece. I don't think these are incorrect, just perhaps incomplete. The piano instructor I had as a child used to make a joke about using purely technical yardsticks to judge complexity/difficulty of a piece: you can tell how hard it is by how much ink is on the page. The implication being that more notation implied more (technical) difficulty.
I challenged her on that being a joke, under the presumption that technical difficulty was the only difficulty that mattered when evaluating a piece. She proved her point by having me learn some deceptively complex songs I'd never heard before, and then comparing my interpretation of them to typical professional recordings of them.
This brought me to appreciate a more subtle form of difficulty, and eventually realizing a profound truth: music isn't simply the execution of a technical skill. It's an emotionally driving art form.
So: Yes, you should consider technical difficulty when choosing whether to play a piece. Try sight reading it. Consider if it uses notation, scales, etc. you have not yet mastered.
But then, listen to the piece with your "piano ears" turned off. Don't think about how hard it will be to play the right notes at the right tempo. Instead, listen to the emotions the musician is imparting as they interpret the piece. Consider the challenge in imparting your interpretation of the piece in a way that will convey emotion to the audience (even if that audience is just yourself).
I think you hinted at this when you said,
??Enthusiasm towards piece??: a lot
You're likely enthusiastic because the piece gives you an emotional response. That's a great sign - so, now, besides just considering the technical difficulty, make sure you consider the emotional or interpretive difficulty. After all, you want your presentation of the piece to also convey emotion - not just be a simple technical exercise, right?
