I wish that children could play instruments. I know many people, myself included, who started learning an instrument at a young age, so it has never made sense to me that it's an off-limits skill until sims are teens.
I also wish there were a wider variety of instruments--including some more "orchestral" kinds like a violin, flute, or harp. Most if not all of the instruments are "form a band" types like guitar and drums.
Finally, I was hoping Starlight Shores might come with another instrument, since it's a performance-oriented expansion, and since I don't want to get Late Night because I'm sure the celebrity system would drive me batty. But I guess my musical sims will all be stuck with the guitar.