Harry's skills are in other thing than sheer magical power (although he did manage to shield charm snape into a desk, and could produce a patronus during his third year) He also held his own at the ministry...

Harry's skills also lie in his leadership (The D.A.) as well as thinking on his feet. He's not perfect, but he can take out voldermort if he plays his cards right.

Again- JK is trying to create some realism here. We can't just have harry win every duel. The books would be downright ridiculous sounding. We also shouldn't expect Harry to gain superman powers overnight either. But what he has... 'Use it well'.

Did anyone else notice that slughorn tells harry that about the felix felices? The same thing dumbledore tells harry about the invisibility cloak, twice... (in bks 1& 6)