Having watched world defense on my server for a while, I notice patterns on what zones get hit a lot, and which ones not to bother.

New Thalanaar: typically some horde farming cloth or doing a quest out there.

The Sea Reaver’s Run:  a daily quest

Farmer Saldeans Farm:  some asshole killing low level quest givers

Honor Hold:  Horde mad about something

Hellfire Peninsula: Horde just outside of Honor hold killing quest givers

Darnassus:  a lot of spam is a world raid, a little is someone you’ll probably never be able to locate.

Darkshire:  Some higher level thinking he’s hot crap.

Halaa:  Lazy PVErs trying to get the mount

Nijel’s Point:  Low level horde who got lost

Astranaar, Lor’danel:  Horde of varying levels messing with Alliance.

I actually fought a BM hunter in Astranaar the other day, he got me though.  They hit hard and heal like crazy.  I could have played better, though.