Ravious, the article's author, defines a hobby element of an MMO as something we may do in our gameplay that's not fun, but aids you having fun in future gameplay. Things like grinding to get that awesome new sword, all the looting you do to make potions for that next big raid, or perhaps just sitting in battlegrounds and getting all the honor points you can take.
Ravious makes one important part at the end of the article -- too much "hobby element" in your game just makes for a sour game. He uses salt for his metaphor in this context. Salt makes food taste better, but too much salt or salting the wrong thing leads to ghastly consequences.