I transitioned from MTT's to cash games last year - not sure everything I here will apply to you, but hopefully something will be useful.
Hardest things for me to grasp in the transition to cash games and why:
1. Stack to pot ratio. Playing medium (around 100 big blinds) or deepstacked (200+ big blinds) was hard for me to understand. In MTT's, the bulk of the time I wasn't playing a deepstack, mostly a shortstack - so I never really got good at thinking about that.
2. Aggression. I played a fairly passive game when I played MTT's - lots of limping, lots of calling based on pot odds
- and I was shoving a lot preflop as a short stack. I wasn't familiar w spots where I should be aggressive postflop.
3. Postflop play and handreading. Playing deep effectively requires good handreading skills, and I hadn't developed that enough while I was playing MTT's (again, alot of the time I was playing as a short stack).
4. Betsizing. In MTT's, I was frequently making what I now think of as small bets postflop - lots of 1/3 to 1/2 psb's. For cash games, I'm regularly making 2/3 and bigger potsized bets and rarely betting 1/2 psb or less.