I don't think my question was answered...
I am starting a brand new character. I have to start in Act 1 right? Should I start on normal, hard, or expert? How does monster scaling fit in?
If you are supposed to start on expert, then what is the point of normal or hard? If you are supposed to start on normal, then when do you switch to hard or expert?
Difficulty makes the game .. more difficult. If you want a harder difficulty try raising the difficulty. The harder the difficulty the greater the rewards. You can play at any difficulty you want, the monsters scale to your level. Here are some numbers for you.
Normal: +0% Health, +0% Damage, +0% Gold Find, +0% Experience
Hard: +200% Health, +130% Damage, +75% Gold Find, +75% Experience
Expert: +320% Health, +189% Damage, +100% Gold Find, +100% Experience
Master: +512% Health, +273% Damage, +200% Gold Find, +200% Experience
Torment I: +819% Health, +396% Damage, +300% Gold Find, +300% Experience
Torment II: +1311% Health, +575% Damage, +400% Gold Find, +400% Experience
Torment III: +2097% Health, +833% Damage, +550% Gold Find, +550% Experience
Torment IV: +3355% Health, +1208% Damage, +800% Gold Find, +800% Experience
Torment V: +5369% Health, +1752% Damage, +1150% Gold Find, +1150% Experience
Torment VI: +8590% Health, +2540% Damage, +1600% Gold Find, +1600% Experience