Once upon a time the arcane tree for mages wasn't particularly good. It had some decent talents for enhancing the other trees, but it wasn't something you'd focus much on. It didn't have a distinct way to play. There were no arcane mages. But there were a whole lot of mages who put points in arcane. Presence of mind combined with a spell power trinket and arcane power and pyroblast... that was something everyone had to try, and then do endlessly through the BG as the only tactic they'd figured out. This is why God gave me grounding totem. This isn't the point.
Disc was also like this: an enhancing tree rather than something someone would actually play. In theory these are all gone. But one remains: Professions.
Professions are the fourth talent tree. Fifth as well. It sounds silly, doesn't it? How can I possibly compare professions to talents?
Imagine a tank skipping stamina talents. Or skipping jewelcrafting and blacksmithing. Both negatively affect their performance, but somehow the professions aren't so strictly enforced. Very few guilds dictate professions, instead they are usually handled voluntarily, sometimes for the benefit of the group, sometimes not.
Or there are the PvP talents, PvP professions, such as engineering. In certain situations an engineer may play significantly differently than a normal player, just as someone with PvP talents may have abilities which you do not expect. Some paladins can break out of stuns without a trinket (talent), stun you without a hammer (profession), survive cliffs without a bubble (profession), and sprint without rerolling a rogue (alchemy or engineering).
What are you doing with your fourth talent tree?