In a car as dodgy and inconsistent as this year's Williams, where even hotly-praised Rosberg struggled (with the exception of two lucky podiums, both earned under very special circumstances), I don't think Nakajima did badly - he struggled about as much as Bourdais, I think. He had a couple of blunders - taking out Kubica at Australia and Alonso at Valencia - but otherwise, he's shown to be not quite that slow. His future in F1 is secured, as Toyota's Japanese card and as a Frank Williams favourite, and I'm still impressed by his opening race at Interlagos, when he actually set a faster laptime than teammate Nico - 4th fastest lap of the race.
All three rookies this season had rather difficult cars to learn on: Bourdais and the ever-twitchy STR2 (which was known for a very nervous rear end), Piquet and the difficult early-season Renault, and Nakajima in the sometimes-just-plain-slow Williams.
yeah, I know Nakajima is the key to the Toyota engine deal for Williams, hence why he is technically a pay-driver (more an engine-driver, lol).
I think now is a good time to review this year's rookie's and how well they've done:
Timo Glock - started off the season pretty mediocre, all the way up to Monaco. After that though, he started to improve drastically, culminating in that 2nd place at Hungary. He's been in Q3 of qualifying several times now and shown some great speed in a car that's not rubbish but not brilliant either. I'd probably make him rookie of the year. Definitely deserves a second year.
Kazuki Nakajima - as has been discussed, okish season, considering the car he's in. Nothing special but nothing bad. I personally don't see many teams being interested in his skills, more his Toyota backing. Not sure if he deserves a second year, but he's getting one any way.
Nelson Piquet Jr - just crap up until today. One lucky finish and the odd point or two gives the false impression he has some skills, but alas he has not impressed me at all this season, although he did have a rubbish car for most of the season, his teammate showed in many races it was capable of beating anything except Ferraris and McLarens. I hope he can prove me wrong that he does have some skill, but he seems to be lucking his way into results not driving. Doesn't deserve a second year yet I don't think.
Sebastien Bourdais - started the season pretty badly and didn't improve till around Silverstone, where he began to set some very quick sector times. He has impressed me a little bit because of the absurdity of seeing a Toro Rosso regularly in the top ten, but seeing both he and Vettel doing it makes me think its more the engine and car really. Still, today he showed he's not far behind Vettel's pace and that he has what it takes to grab positions in qualifying and race. His only issue seems to be reliable finishes and luck. This season's sob story if there ever was one. Deserves a second year.