I find that if I really know a fight (like done it 8+ times), my damage as BLM goes up substantially.

For example, on Demon Wall, the first time I did it, I felt that I was dragging down the group. Now I can put on damage on par with Archers. I'm not sure if it's the same with other classes, but with BLM I feel this is particularly important.

You learn:
1. When to move (saving miscasts)
2. When you can raging strikes(getting better fire cycles)
3. When you should use your procs (a well timed shortly saved proc can give you more time to move)
4. When to swift cast (same as 3 but on some batters it might be a good idea to save swiftcast for flares/fire 3s to get into cycle earlier)
5. When to flare/convert or just flare (you never want to get caught with your pants down w/o mana, but flare does do tons of damage)
6. When you have to unload everything because a break is coming up
7. When you're going to be interrupted because of unavoidable AOE (use a proc or sure cast)
8. When to kite/scathe or ice in mana regen cycle.

Plus probably a lot more. When I'm going in without actual battle experience, I feel like I'm doing about 60-70% of my full damage most of the time.