Ok, now I've seen the film I can understand, though not necessarily agree with, why some people are unhappy about some of the changes and additions made by Peter Jackson.
The TT is a much more difficult book to film than the FotR. The first book is much more linear in structuire, whereas the TT requires substantial directorial verve to knit all the disparate storylines together in a way that does not seem disjointed. Having just re-read the book prior to seeing the film (something I deliberately avoided last year) I don't think that the overal 'feel' of the book has been lost. Cerytain aspects have been enhanced whilst others have been played down.
There is less about Merry and Pippin that I would have liked and the Ents, whilst well shot (yes, they even plant their toes down first when walking, just like Tolkien specified) I did feel that some of the unhasty majesty of the treeherds had been lost - maybe to be restored with the longer DVD version next year? While there is less of Frodo, Sam & Gollum as a percentage of film compared to the TT book, this part has been pared down well and in some respects made more inteesting and thoughtful - e.g. playing up the tensions between Sam and Frodo caused by the Ring and counterpointing this with the dichotomous personality of of Gollum/Smeagol.
In general I feel the film played down the hobbits and enhanced the role of Aragorn as troubled and lovetorn hero. I see no problem with this in terms of making the film appeal to non-Tolkien fanatics. Eowyn was well-portrayed, coming across as brave yet tragic, just as she does in the book. As for Legolas and Gimli, are these two the new Batman and Robin? Gimli's comic lines were quite funny at times, at least to a veteran roleplayer used to dwarf jokes on a Friday night round the table. Whether they were in keeping with the character portrayed in the book is another question of course, though having comedy play a part even in black tragedy is an old trick (Shakespeare used it a fair deal) and is none the worse for it's inclusion here.
Legolas, hmm, a skate-boarding elf? OK, ok, so it was an uruk-hai shield, but I winced at bit at that point in the film - the only time I did so really. His "you're late" line to Aragorn was well delivered and (I thought) saved from being cheesy by being delivered in Elvish. Not sure why Aragorn went over the cliff - perhaps to enhance the Eowyn storyline and to find a way of rbining Arwen in by contrast?
I could go on, but I won't. The fact that I could sit here and type or talk about a film seen late last night when I was already very tired, speaks reams about the film. Overal I probably didn't enjoy it as much as the Fellowship, but then a) the TT was my least favourite book (my favourite is still to come) and b)I've seen the Fellowship a number of times and appreciate the film more. Perhaps, as Tolkien might have said, this film will grow in the viewing? 
It's better than James Bond, Harry Potter or any other film I've seen since the Fellowship. Go see it. Again.
Colin