Diaby's always been potentially a very good player, just been hampered by injuries. If he can stay fit you can see why Wenger had no problem letting Song go, it's just a gamble with Diaby's fitness (much like we're doing with Cleverley & Anderson)
He's hampered by looking like he could achieve something, but never doing it. Gervinho is Diaby Mark II, Diaby started on the wing, had a very good ability to hold on to the ball, run down the wing and ultimately do exactly nothing at the end of it
Injuries stopped him having a full season as Gervinho had, for the fans/manager/pundits to be able to say "he's had a full season, or three, injury free with no more excuses, he's just not good enough", while its 2 games here, 5 games there, people put his final ball and not actually being involved enough down to, "maybe he needs some form/sharpness, maybe he's not good enough", and that maybe has won him years of extra pay for being not good enough.
He's had MANY performances like that one, many better performances, then been so horrifically poor the week after its a joke.
Diaby, 2 goals against Pompie, everyone raving about his performance(in reality that performance was actually less good than the one against Liverpool, just better shooting), then next league game, scores a ridiculous own goal against Utd, was terrible throughout the entire game.
If you regard Liverpool as a top top side currently that is one thing, if you rank Liverpool's current quality then you'd say Diaby had an ok game in which nothing he did really made a difference against a very average league side, he's always been capable of that.
Where Song/Diaby/Denilson/Walcott have cost us is in 9/10 games against the difficult sides in the league.