I have yet to do world completion on any character and I know exactly why that.
It's because I want to play the racial starter zones as the race in question. Dunno why, call me a weirdo, it just seems right.
To then come back and repeat those zones on a different character is frustrating. Why isn't world completion account-wide? It's technically simple to accomplish (count each time a zone is completed, calculate how many world completions have been achieved) and would greatly reward players that play a lot of alts.
Potential negatives "What if you delete a character?" Let's not over-complicate things. Keep it the same. Deleting character deletes all progression towards World Completion.
Or track it with infinite individual achievements for each zone and create a meta that tracks World Completion.
It really can't be that hard.