I don't exactly know how the IATA assigns codes or how airlines jockey for IATA codes, but for some reason you never seem to find too many airlines that have "nice" IATA codes.
BA works out extraordinarily well, as does VA for that matter, and NZ, AF, AA and RJ.
QF is one of those "close" ones and a lot fall in this category: CX, SQ, MH, PR, KE, JL, SK, JQ and so on. (EI just works if you think about where the airline comes from

)
Then there are those "removed" ones that keep you guessing: anything that starts with a number, AY, DJ, JK, JJ...
On a more creative note, perhaps they chose DJ because the abbreviation sounds cool and party-like. Besides, what trendy airline would want to use an abbreviation that shares its likeness with a shoddy beer?
VB is currently a free IATA code according to Wikipedia....