I find that Offensive skills level up much faster because of the 50% damage penalty the player suffers from on Master (As opposed to Adept)
On the flip side, Defensive (Particularly Armor and block skills) level up slower, because each hit, especially by near-level enemies is incredibly dangerous. A Marauder's Warhammer power attack at Level 24 is Liable to one-shot a player who had selected Health for 75% (280Health) of their level-up bonuses.
People say Conjuration is most powerful, but this one knows that not one, nor many can stand up to a Master of Illusion!
Amen, I'm taking my Nightblade through Thieves Guild and Dark Brotherhood right now, I finally got expert spells and it's a little sad. About the only creatures that stand a chance are Falmer and some Automatons (Master class Centurions and Spheres) since they are too high level to frenzy or fear without Vampirism and/or Alchemy abuse.
And whatever I can't use Illusion on, I can sneak attack. Though, when my cover is blown (Particularly against falmer which don't respond to Invisibility) it can get pretty nasty. Good thing the Nightingale blade has that absorb HP effect.