This is something that has bothered me for some time. I just am having trouble finding some evidence for it. I'll search more but if others have some feel free to post it up.
But basically the situation is this:
- Indoor zones require you have line of sight to begin and end most all detrimental spells.
- Outdoor zones did not require you meet a line of sight check beginning or end
The bold currently isn't working on P99 as it did in classic.
This is why Unrest was especially dangerous, where hags would nuke you through the floor.
Also why things like trees, hills or small buildings/walls/houses did not prevent spell casting. You could go behind a guard tower or merchant house and aggro the mob by casting a root, snare, fear, DoT, nuke... Whatever, without actually having line of sight. Or much more importantly, if you are casting on a mob and it is just cresting a hill, your spell won't be interrupted because he is still behind that hill.
All that mattered in an outdoor zone was your range to target.