Yoda Is Your Father said:
If the bloke playing Kirk done a Shatner impression everybody would laugh their arses off and you lot would moan.

Also, I'm sure he was directed on how to act it - go easy on the guy man, shit.

I agree. Besides, if I wanted to see a Shatner Kirk, I have countless hours of that already available.

In fact, Pine said he intentionally stayed away from doing a Shatner impression. Here's an interview in Variety where he addresses that very subject;

http://www.variety.com/article/VR1117994644.html?categoryId=3289&cs=1

"They're that iconic. They left that indelible a mark on the social psyche," Pine says of Kirk, Spock and the rest of the crew.

In jumping full throttle into cramming sessions with "Trek" lore encyclopedias and watching the original series, however, Pine decided midway that he was doing himself an injustice.

"All I was doing was heaping upon myself an incredible amount of responsibility, weighing myself down with 'You must remember to act this way' and such," explains Pine, taking over a part whose mannerisms, as embodied by William Shatner, have become instantly recognizable. "That's when I was really able to let go and accept the parameters that J.J. set forth in the beginning, which was to bring what was special and unique about us to the roles."

That meant embracing Kirk's descriptors -- charming, funny, leader of men -- rather than a predigested image.

"Just try to be human," Pine clarifies. "I got to do everything: yell, shout, cry, laugh, take charge and be vulnerable. I pulled the lucky straw."