Here's just a short list of what I found searching the site for 20 minutes. Most of the examples, people had to take a screen shot of what they were explaining or just put the note letters instead of visualizing the music.

* http://music.stackexchange.com/a/1166
* http://music.stackexchange.com/a/40222
* http://music.stackexchange.com/a/22242
* http://music.stackexchange.com/a/16999
* http://music.stackexchange.com/a/33950
* http://music.stackexchange.com/a/14783
* http://music.stackexchange.com/a/583
* http://music.stackexchange.com/a/23977
* http://music.stackexchange.com/a/4945
* http://music.stackexchange.com/a/6919
* http://music.stackexchange.com/q/17632/7222
* http://music.stackexchange.com/a/12074
* http://music.stackexchange.com/q/24355/7222
* http://music.stackexchange.com/questions/43428/playing-arpeggio-on-black-notes-with-white-note-third
* http://music.stackexchange.com/questions/32787
* [How to correctly invert non-compound greater-than-octave intervals?](http://music.stackexchange.com/a/43544/72220)

And from the first page of questions :

 - [http://music.stackexchange.com/questions/43355/a-minor-scale-has-different-notes-when-descending][1]
 - [http://music.stackexchange.com/questions/16992/which-scales-are-used-by-tom-morello][2]
 - [http://music.stackexchange.com/questions/43363/why-do-the-notes-of-melodic-minor-scale-change-when-you-play-it-in-descending-or][3]

There were another 3 on the first page that might have benefited from being able to write notation, but these were the clearest cases.


  [1]: http://music.stackexchange.com/questions/43355/a-minor-scale-has-different-notes-when-descending
  [2]: http://music.stackexchange.com/questions/16992/which-scales-are-used-by-tom-morello
  [3]: http://music.stackexchange.com/questions/43363/why-do-the-notes-of-melodic-minor-scale-change-when-you-play-it-in-descending-or