I've played a lot of PVP in MMOs and enjoy FFXIV's. However, I believe an adjustment of line of sight and damage taken is needed.

Based on playing Wolves' Den and Frontlines, it is my assumption that the ability to cast a spell/ability is dependent on the relative position of the player who is casting it. This prevents the issue that used to plague WoW that you could be on top of someone on your screen but received a "target is too far away" message.

I believe a second positional check is needed to register damage taken. That is, if you run behind a wall and start casting a spell, that break in line of sight should prevent any damage that you take thereafter (even if their spell/ability was cast successfully). At this time, you run behind a wall, start casting a heal, and are interrupted. This goes for any class attempting to mitigate damage via LOS.

I think this change would improve PVP in this game tenfold and allow people to take it more seriously.