I've often thought that if there's space for something to be reformed in musical notation, it's the fact that different wind instruments are notated in different keys[1]. I realize there are historical reasons, but it just seems like such an artificial barrier between musicians in a modern band or orchestra.
It's a standard part of musical training to be able to read a part written in either concert pitch or in your instrument's pitch, or even to be able to transpose on sight into any key. It's not easy but learning to do so pays off when you're on a gig and the singer insists on playing Lush Life in B natural.
We can debate how large an obstacle it is, or whether learning to overcome it is valuable, but clearly it is there, even if it's just an annoyance. I just think it would be really nice for (e.g.) a clarinetist to be able to sub in on an alto sax part on sight without having to go the extra mental work of "Ah, right, up a perfect fourth --"
This is mildly annoying for composers, but it doesn't really matter for the players. They play the notes they see on the page. I suppose if they have perfect pitch it might be a bit jarring.
I play mainly clarinet (which is written Bb transposed), and the transposition thing is not a problem at all. It's the relative intervals that matter anyways. I don't feel it creates a barrier when communicating with others in the orchestra (we talk about concert pitch anyways), and not having to count five extra staff lines makes up for the small inconvenience.
This makes it much it much easier when switching between different instruments of the same family - a clarinetist (when playing any clarinet part) associates one note on the staff with one fingering. If all instruments were in C, the player would need to associate the same dot with multiple different fingerings, depending on the family member being played.
http://en.wikipedia.org/wiki/Transposing_instrument