He was "memorable" but "not the greatest Dane" with a "spirited but unripe" rendition, Mr Letts said.
Paul Callan in the Daily Express said Tennant pitched his Hamlet "somewhere between One Flew Over the Cuckoo's Nest and The Goon Show".
He added that Tennant relied too much on "quirkiness, pulling faces and various funny voices" to reflect Hamlet's descent into madness.
But Mr Callan said his "near-whispered delivery is sensitive and polished".
'Engaging' Hamlet
The Independent's Paul Taylor gave the play four stars and called Tennant's display "an extremely captivating performance".
He said the actor was "adept at most aspects of the role but he excels when the prince becomes a prankish provocateur".
He added: "I rate Tennant very highly, but I wouldn't put him in the absolute front rank of contemporary Hamlets."
Michael Billington from The Guardian also awarded four stars to the play.
I've seen bolder Hamlets and more moving Hamlets, but few who kept me so riveted throughout
Benedict Nightingale, The Times
"He is a fine Hamlet whose virtues, and occasional vices, are inseparable from the production itself," Mr Billington said.
He said this was "a Hamlet of quicksilver intelligence, mimetic vigour and wild humour", adding Tennant was an "active, athletic, immensely engaging Hamlet".
Benedict Nightingale of The Times said the play's "fluent, pacey, modern-dress revival" of Hamlet gave Tennant the chance to shine "and, praise be, he seizes it".
He wrote: "I've seen bolder Hamlets and more moving Hamlets, but few who kept me so riveted throughout.
"Tennant is restless, curt and mocking when he needs to be, affectionate when he can be, and, apart from an occasional tendency to gabble, pretty impressive."
'Flattering'
Tennant drew more than 10 million viewers to the series finale of Doctor Who last month.
He will star in a Christmas special and another three specials next year.
The actor, who played Romeo at the RSC in 2000, has said of appearing in Shakespeare's great tragedy: "Hamlet is often regarded as the acme of acting to test yourself against, which isn't a particularly helpful thought, to be honest.
"Of course, it is very flattering to be asked to do that role because of everything that is attached to it.
"But once you get into rehearsal you have to relinquish thoughts like that and just try and tell the story.
"It's still just a play and you can't start approaching it in a different way than you would approach any other role."
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