Do NOT make the same mistake as FFXI.

[Demacia]

This is, unfortunately, completely unavoidable in MMOs. The only way to fix it is to embrace it and balance against by making content that's only manageable by each class. FFXI has the same exact problem -- if you want to get something in Abyssea you have to have WHM/BLM/NIN/THF/WAR, and often times even if you have one of those jobs yourself, you end up needing another one of them because no one else in your group has it. I just finished spending my entire weekend getting /BRD for BLM, just to have every group I tried to get into tell me "Sorry" because I didn't have WHM90 to take with them.

Its frustrating -- but the only way to avoid it is to either make everything 100% solo-able, or to cater to it by introducing multiple methods of getting things done that require each job specifically. The only way to make people want to bring every job is ot introduce a mob for every type of damage -- but then it becomes impossible to do anything unless you have one of every class, which is an equally daunting and frustrating task.

Honestly, I never felt that FFXI truly had that issue UNTIL abyssea was released. The weakness trigger system absolutely ruined job diversity.