I've been going back and forth in my mind about LF. As already noted, LF's character has been changed in a couple of ways. The question is, why? A few tentative thoughts:
1) Backstory: we know that LF comes from somewhat humble beginnings, so how the heck did he rise to the position, power, and wealth that he now enjoys? HBO's answer: pimping. And this is why he is and will always be an outsider. He may well be successful in many other financial enterprises; he may well be so much more than a proprietor of brothels; but it's the brothels that in a sense define him. And so we need to see what kind of brothel owner he really is--cruel, ruthless, mercliess.
But why do we need to see this more so than the book reader? He actually didn't rise to his current level of power thru pimping in the books, he rose in great part because of Lysa Arryn, which we find out at the end of Storm of Swords. She convinced Jon Arryn to promote him up thru the court to his eventual title of Master of Coin, and he had the ability to make the best of those promotions.
2) Credible motivation: one of the weakness of Book-LF is motivation. Why does he put all at risk? Why does he attempt such an incredibly elaborate, dangerous scheme? Is he trying to take over the world, or is he trying to destroy the world? Does he seek power or chaos? I don't think we know, and it doesn't really matter, as we are all enthralled with this evil genius who would be comfortable in a James Bond novel. Who cares what motivates Blofeld. Who cares what motivates Professor Moriarty. Evil geniuses like this are larger-than-life. But the HBO writers evidently decided that if they were going to make their LF both interesting and credible, they needed to make a bit more human--less the criminal mastermind who never blunders and whose motivations are irrelevant.
I don't think Littlefinger is motivationless in the books. He is motivated by his lack of noble birth which made him an unsuitable match for Cat, and that motivation has inspired him to gain the title of nobility and to stick it to the nobles that looked down on him. He may have even deeper motivations and no one knows what his end game strategy is yet, but personally I think that is part of what makes him interesting. Besides the person who needs everything spelled out for him isn't likely to spend alot of time wondering what Moriarty or Blofeld's motivations are in the first place, and the person who thinks about such things is more likely to appreciate the subtlety and mystery of wondering.
3) Interest: How interesting was LF really in the first two books? Those of us who have read all the books marvel at the man's genius and audacity, as revealed after he abducts Sansa and murders Lysa much as we admire a Professor Moriarty; but Moriarty is only interesting to those who know him to be a criminal mastermind. To the rest of the world he's just a university math teacher, hardly worthy of notice. The HBO writers thus faced the problem: how do we make LF interesting during the first two seasons for viewers who have never read the books? And so they decided to reveal his unsavory side early on and to make him vulnerable (his confrontation with Cercei). In other words, they added drama and depth to his character. The cost is that he is no longer Moriarty. He's no longer larger-than-life.
This may be how the TV writers view it, but I strongly disagree. How interesting was Benjamen Linus when we first met him in LOST? How interesting was Verbal Kint in The Usual Suspects? Part of the payoff is the characters starting off as less interesting, or threatening, and becoming much more than they originally appeared. But the TV changes don't add drama or character to Littlefinger in my eyes. They telegraph his villainy and make one question how someone could be so dumb as to provoke Cersei for no gain in public and, yet have the forsight and patience to execute the plans that are revealed in books 3 and 4.
Now I think they do have to add some scenes of some sort to keep the actor happy so he has something to do in the early seasons. But the writing for those scenes could be more true to the book's character, much like Varys who is similar in the level he is playing the game and what we do or do not know about him at this point. Honestly from season 1 I think they really missed an opportunity when they cut Loras's plee to hunt the Mountain instead of Beric Dondarrion. The follow up to that scene was Littlefinger and Sansa running into each other and Sansa complaining to Septa Mordaine about how Ned should have sent Ser Loras because heroes kill monsters. Littlefinger responds "Well, those are not the reasons I'd have given.. "Life is not a song, sweetling. You may learn that one day to your sorrow". Which is a shame because I really believe that is the core theme of Sansa's story line so far, and is a line that Littlefinger references again in Storm of Swords to her.