Ok, so it was a slight exaggeration but pitchers do take forever to throw the ball, and baseball is a very slow-paced game.
(More than a "slight" exaggeration.)
Depends on the pitcher. Steve Trachsel is very slow, Mark Buehrle is very fast. I don't really get why people have such a problem with there being some time between pitches. I mean, do you want the pitcher to throw rapid-fire? That would be stupid. Time is taken between pitches for several reasons:
1- So the hitter can reset. Hitting a baseball is very, very difficult, you can't just go up there and hack away. Guys work for years to perfect their swing, and with that comes a unique batting stance. Watch a game and you'll see that Kevin Youkilis in the box looks completley different than, say, Moises Alou. Maintaining the stance and gripping the bat can make you cramp up, as well. Then some guys are jusy weird and love their rituals. Nomar Garciaparra
must step out of the box and reajust his gloves after every pitch.
2- So the pitcher can reset. Pitching is very unnatural and puts a ton of strain on the body. These guys aren't just throwing the ball back and forth; different pitches require the ball to gripped in a certain way, apply a certain amount finger pressure, throw with the arm at a particular angle, etc. A lot of guys need to snap their shoulder or elbow in particular way to make a pitch move the way he wants, and that can put a lot of stress on the joint. They need to make sure they repeat their motion exactly (move the same way, arm at the same angle, feet landing in the right spot, etc.) for every pitch, otherwise they could seriously hurt themselves, or just not pitch effectively. Not to mention how much it takes out of someone; throwing the ball at ~90mph takes a lot of energy. Also, there are rules dictating the placement of a pitchers feet before he throws a pitch and how he prepares to throw. A guy can not just stand there and toss, they have to place a foot on the rubber, face the hitter and have a distinct "pitching motion".
3- Signs. Probably the biggest thing is that, between pitches, the pitcher and catcher must decide what to throw. The catcher doesn't just recieve the pitch, he calls for it, and studies the other teams hitters to figure out what would be the best pitches to throw. If you throw a high fastball to Vlad Guerrero, he's probably hitting it out of the park. Throw it to Juan Uribe and he'll swing and miss. There's more to it than that, even; pitch selection also depends on the situation. Most pitchers won't throw a strike with the count 0-2 (Jon Papelbon tried that the other night and Alex Rodriguez put it in the seats). The catcher is also telling the guys in the field what to do, depending on the situation (the hitter, the count, outs, the score, who's hitting, who's pitching, the inning etc.); play deep, play shallow, play to one side, etc. If they are planning a pitch-out or a pick-off (when the pitcher throws the ball to a base instead of homeplate, tying to catch a runner leading off the base), that is often a decision made between the pitcher and catcher. The hitting team is flashing signs, too. They are communicating and deciding if someone is going to try and steal, will their be a double steal, is the hit-and-run on, is the hitter going to bunt, etc.
That's about it.