IIRC the Japanese in WW2 created a jet engine from a single photo of the German one- one of the greatest reverse engineering feats yet.
I think it would be rather difficult to create an effective defense to an alien weapon by just observing it's effects in battle- having a sample or more intel about the principles behind it saves a lot of time. In the Best of Both Worlds Admiral Hansen spoke of how long it was taking the Federation to develop new weapons to fight the Borg- this tech tree resulted in the NX-Defiant eventually. War produces innovation with extreme motivation, but when you are attempting to counter an enemy's weapon technology it never happens as fast as you would like and then deploying it takes a lot of time as well.
The Breen were a one trick pony in battle from what I have seen on screen- they had the courage to strike Star Fleet Headquarters but after the initial strike were pretty much wiped out. In battle you see their wonderful asymmetrical ships usually getting blown up in one shot, it is only if they can fire that Energy Dampening weapon before they get wiped out do they contribute to the fight.
One disappointing thing about the Dominion War as we saw it on screen and the choices on this thread- the War was against the entire Federation, not just Star Fleet. I would have liked to have seen some Vulcan and Andorian craft in the battle group, sort of like the armada Captain Sheridan raised in B5 for the final Shadow War battle.