A lot of really harshly clashing tones that don't resolve all throughout this piece. Examples:
:11, poor resolution of higher voice tones.
:15, poor resolution of higher voice tones.
:17, this would be okay if it shot off to something else but it doesn't... it goes right back to the same atmosphere.
:23, clarinet A# is a bad resolving tone.
One needs to understand regular classicism to understand neoclassicism, there's just a lot of modulations, rather than a proper sustained line through the whole thing. Prokofiev's Classical Symphony is a must for you to listen to.
There's also a lot of repeated material which does get tiring after a little bit. Think of it like a rondo where the A theme is there and comes back, but isn't overwhelming.