The reason that Reacher is such an intriguing hero/anti-hero character in the books has to do with his size and his taciturn nature. He is tall, heavy, muscular, and has some very strong quirks; he doesn't own anything beyond what he carries with him, he doesn't have any real ID (until later in the series, anyway), he can't drive, and he doesn't use toothpaste (or floss or mouthwash)-- he simply uses a toothbrush, which is one of his few possessions.
His size is almost a character itself within the books. He rarely meets someone his own size, and he uses his size and physical presence to his advantage in the novels: frequently he can stop a fight from happening just by being so physically intimidating; sometimes he allows people to assume that because he is large, strong, and quiet, he must be stupid or slow; and he uses that great size and his attack-first mentality to end a fight before the opponent knows it has even begun. Not to mention how many women are attracted to him because of his size and physical features (even though he is not an attractive man, his size, confidence, and physicality make up for it).
Another big issue that comes up a lot in each of the books is his inability, and lack of desire, to drive. He comments frequently that he breaks cars, grinds gears, and just isn't very good at any aspect of driving. He is always giving the keys to someone else to drive him; those close to him just assume he won't be driving. The few times he is forced to drive in the novels, bad things tend to happen.
Reacher, in the books, is also so physical that many of the fights don't last more than a couple of hits. Reacher believes in hitting first, hitting hard, and ending the fight before it begins. He does this because he understands that the longer the fight goes on, the more chances to get seriously injured he has. Reacher is no dummy; he wants to stay as safe as possible and without injury. So, he will head-butt a person before they are expecting it. Or he goes for a debilitating one-punch/blow type of hit, like to a nose or the neck. When fighting multiple enemies, he will try to kill or maim someone seriously in one blow so he can turn his back on them and go after the next. He is a whirlwind of force and power, not to be taken lightly.
When Tom Cruise cast himself in the role of Jack Reacher in the movies, fans of the series were a bit surprised. Reacher is 6'5", Cruise is listed at 5'7" (some say he is closer to 5'6" or even shorter). Forced to make the best of an odd situation, Lee Child made a statement about Reacher's size being "a metaphor for an unstoppable force" and that Cruise's acting and status as a star could fill that gap. I'm not sure any fan really bought it or thought that wound up true in the first film. While Cruise can play bad-ass and has real acting chops (although he hasn't really used them for awhile now), he just doesn't come across as the physical person that Reacher just is.
The first movie contained multiple fight sequences, and all of them involved fairly complex choreography in which Cruise fought both single and multiple entities. Because no one would believe his little body could take people down in one or two blows, the fights were lengthy martial arts dances that just didn't fit with the character of Reacher, whose fighting technique is military close-fighting and street fighting techniques.
That first movie also showed Cruise in a complex, interesting, and dynamic car chase sequence, where Cruise's Reacher showed exceptional skill at driving. A total departure from the books in every way. The trailer for movie two also shows a car chase sequence in it, with Reacher once again behind the wheel.
While they tried to keep the physicality in place, Cruise came across somewhat laughable when he threatened people in the first movie. In the trailers for the second movie, I find myself smiling once again when he threatens people. When a 6'5", 250 lbs monster of a man threatens you it is simply different from when a 5'7" guy threatens you, even if that smaller guy knows martial arts and can kick your ass three ways to Sunday. On screen and magnified, it just doesn't look or come across the same way as in the books.
Lastly, Cruise is a pretty boy. Reacher isn't. As Cruise gets close to and seduces women on screen, they are reacting to how pretty he is, not to his sheer .... animal physicality and confidence. Again, it is believable that Cruise can get the woman because he's, well, Tom Cruise. In the books, it is almost always Reacher's actions that overcome a woman's lack of interest; he is confident, kind and courteous (in a military sense as well as just a "nice guy" sense), has a raw physicality, and is protective toward them.
I will probably not see the second movie in the theaters due to these changes, but likely will watch it on video later. While I'm happy that the character has made it to the silver screen, the changes to the fundamental nature of the character are off-putting to me. I didn't actually mind the first movie and thought they did a pretty good job of bringing the book's story to the screen. However, I was constantly taken out of my immersion in the film by Cruise's size and totally different take on the character. I'd like to see a movie with a more faithful-to-the-character portrayal of the character. I can see someone like an Alexander Skarsgard playing the role, as he is about the right size (6'4") and can act.
ADDENDUM (1/7/2017)Finally watched the second movie. Beyond having some of the same characters, locations, and general plot beats, the movie looked little like the Jack Reacher novel it is based on. It wasn't a bad movie, it is just not Reacher. They have to rewrite and write around Cruise being so totally unfit for the part that it snowballs into changing large sections of the plot.
What's funny is that John Wick makes for a better Reacher movie than either of the two Reacher movies. While still not the size of Jack Reacher, John Wick has the intensity, economy of movement, and deadliness that a Reacher fan should enjoy.