I am a HUGE fan of gameplay mechanics and class interaction. Simply put, if you can find time interested in just figuring out your character, by customizing it over a period of time, you'll find endless extra hours playing that character hundreds of different ways in your other gameplay.

I found Anarchy Onlines twinking system to be the best for this, its based around buffing with spells and gear, you can equip some reeeeaaaalllly high level gear by simply stacking buffs and stat buffing gear, but instead of just doing that, you can put on other gear that buffs stats, effectively climbing the stat ladder. Ive done this for hours/days tweaking a character, specifically to either own a dungeon or to level insanely fast/solo.

Now that I play World of Warcraft, I basically know my classes best spec's for their jobs and, its not nearly as interesting allocating points/customizing my character, glyphs are the same way. The only real customization is through the acquisition of gear.