"It will be great" - in the future.
1) "it" is going to be great. "It" is definately great.
2)"It" is definately going to happen.
-certain, confirmed
"It would be great" - in the future.
1) "it" might be great, but might not.
2)"It" could happen, but is not certain.
-uncertain, unconfirmed
I hope this helps, It might not.