Berdych, Fish, Almagro, and Monfils all have significantly more powerful serves than Nadal.
And I quote ClayLover:
Weak FOR a top 10 player and weak COMPARED to other top 10 players are two seperate things.
Nadal's serve is INDEED weak for a top 10 player. But there are two reasons why it hasn't been fully exposed -
1) Left handed advantage
If he served right handed, any half-decent returner would've eaten it alive.
But you can't compare lefty serves to righty serves that way. It's comparing the color of apples and oranges. You have to compare what's in the apple and orange, what's in the serve. Lefty serves don't have to be as "BOOM BOOM POW!" as righty serves, and they tend to have a lot more spin (from what I've seen).
Nadal's serve is a solid lefty serve. It's not Roscoe Tanner, but it gets the job done. And it gets the job done well.