I've always said that an MMO is literally the most complicated piece of software one can make. Take every single problem that exists in software engineering, and you have it in an MMO.
A) Every problem from a normal game.
1) Resource streaming for an open world.
2) Particle system running on 5 year old commodity hardware
3) Physics system to handle projectiles (Even if it's not havok you still need something for the characters falling from the sky.)
B) Every problem that a business app would have.
4) High availability clusters
5) Billing systems
6) Massive databases
7) Customer Support back end
8) Call center support
C) Every problem that 'internet companies' have
9) Latency kills
10) World wide datacenters mapping 1:1 and 1:many architecture pieces
D) Some nice unique problems for MMOs only
11) Cross server object replication
12) More hackers targeting it than they would some banks.