LoS check is wrong - outdoor objects like trees or warsong tree stomps works well, but also fences and most other outdoor objects also shouldn't block casting. Atm even small flovers or torches prevent casting.
http://www.wowwiki.com/Line_of_sight?oldid=304289
Terrain (that is, dirt and trees) only block Line of Sight indoors. All dirt and most trees do NOT block spells or abilities when fighting outdoors.
Videos:
http://www.youtube.com/watch?v=y8GGLXnaFGU
8:54 - hunter attacking through tree stomp, 9:28 - hunter attacking through fence
http://www.youtube.com/watch?v=j4tDfYcjZ_w
5:04 - hunter attacking through tree stomp, 6:42 - hunter attacking through fence
http://www.youtube.com/watch?v=LPtg80s26iI
2:25 - hunter attacking through tree
http://www.youtube.com/watch?v=GSW8TcZ9URc
0:24, 0:37 - hunter attacking through fence
http://www.youtube.com/watch?v=bytAiTenKY4
7:25 - Zachary polymorph through tree(also many other moments in this video and his others, but they are too long to count all of them, if some1 interested - can see themselfs)
I hope this will be helpfull to make blizzlike LoS check.