Writing an octree is definitely part of coding an MMO. It's a piece of software, of which an octree or something similar is a fundamental part, thus it totally fits. If this is too slow-paced or ground-level for your tastes ... avoid game development, it just gets worse; more complicated, more pedantic, more detailed, more time spent not actually creating game logic.

However, I'm also cheesed at the misuse of the term MMO, Minecraft is pretty close but the lack of a centralized game world in any way makes it unlike most MMOs, and by extension doesn't really fit the term. It is merely a persistent multiplayer construction set. N8 (Neverdaunt) or Wurm Online (which notch had formerly worked on), for example, are similar construction-oriented games that definitely fit the MMO moniker because there /is/ a centralized persistent game world. Dealing with the side effects of such a architecture does make things an order of magnitude more complicated.