I don't think its fair to say that Smith is basically a younger Troughton - the latter was relatively more caculated than Smith, who's certainly like a mad scientist. And as has been pinted out, Smith seeks approval while Troughton was content to go unnoticed.
I think that Troughton did set the precedent of the Time Traveller & Co. to the show. He was the lead that did the unusual, the unexpected that the companions around had no idea about. Doesn't mean the given interpretations weren't their own man - far from it. That the Tenth has elements of Four and Five is, in fact, a nice addition of nostalgia towards the character himself, and a real sense of personal history.
And to add, yes, I will miss Matt Smith. He's had the unenviable job off following after an immensly popular Doctor, and made the part truly his own. I wish he'd have stayed, but at the same time I look forward to the new Doctor - thats how it is with DW, after all.