As horn players we see a lot of horn parts written by composers who don’t really know how to write for the horn. First, the point of this post: if you are a composer and you are not sure about your horn writing, talk to a horn player. We really can be friendly and helpful, and can help you avoid problems like these:
Too high. Yes, sure it sounded great on the midi, but really horn can’t play hanging around above the staff for very long. There is a heart of the range that sounds the best; get it defined clearly and horn parts will sound great. Very high, technical parts will sound much better on trumpet or saxophone.
Too much bass clef. At the opposite extreme you have parts that are too low. Part of it is the composer did not realize that horn players can read the full octave below treble clef in ledger lines easily. The larger issue is why are they in this range so long? Because the composer must be totally afraid of the horn. Again, get familiar with what is the characteristic heart of the range.
No time for mute changes. These are very annoying. On other brass instruments players can have a free hand to make a mute change with no rest. But not on horn. We have to take the mute out then put our hand back in the bell. At a minimum we need a couple beats of rest and would prefer more. Sometimes you can get the same effect by asking for stopped horn instead of muted, and there are a number of examples of famous works where horn players normally play stopped instead of muted as marked because there is no time to remove or insert a mute.
Stopped notes in too loud a context. The final item for now is related to the above. While I certainly can imagine a stopped horn sound soaring over a large orchestra, in reality the loudest we can play stopped horn even if marked and playing what feels like FFFF is about MF out in the audience. Stopped horn does not project, use it with caution. A stopping (brass) mute helps with a bit more volume, but we have to have time to put it in.
This list could go on and on. There are great resources for composers out there, especially the wonderful publication Extended Techniques for the Horn by Douglas Hill, check them but also just ask a good horn player and get some feedback, it will make your music more successful.