There are a few other quests in-between of course, as well as particular random quest events that occur along the way. To take a moment, this is also one of the ways Blizzard is making the game more dynamic, by throwing in fun random events and quests throughout the world that don't always happen. Maybe you need to slay a resurrected ghost or pass a jar of souls event, or find a particular character or monster. Your character also comments on various monsters and locations.
I remember reading somewhere that Blizzard wanted to include more story and lore in the game and expand on Sanctuary, which for those who are not familiar is basically earth in the world of Diablo. It's viable for both angels and demons, which is why they are always fighting over it and you end up saving everything and everyone as the hero. Blizzard has done a good job of this in terms of the writing, characters and quests. It does make things more interesting without getting in the way of hacking monsters to little bits with my axe. Still, if you want, you can talk to characters and have some revealing conversations. For example I found out my barbarian's father was killed when he was "but a pup" and then proceeded to strangle his father's killer to death with his bare hands while screaming in rage. Violent, but a nifty bit of character insight. There are also "Lore updates" from a variety of characters, including Cain, which you can click on to hear a voiceover on a new monster or area. Again, purely optional, but a nice touch.
Those hoping for a true demo may be disappointed by the beta. Your max character level is 13, and you can "finish" the beta in just a few hours. The only annoyance here is that there isn't more of it, and all said and done Diablo 3 seems to be just as addicting as previous games, which is a good thing. Still, you can play every class and getting to level 13 takes several hours if you want to grind it for every class. As far as classes go there are 5, and we have the standard Barbarian as mentioned. The other four are the Witch Doctor (think modified Necromancer), Wizard (Sorcerer), Monk (no direct comparison), and Demon Hunter (Think a cross between Amazon and Assassin). I tried out every class for a number of hours to get a feel for each one. Classes are much more unique in Diablo 3 in that they all have different ways they power and use their skills. Barbarians for example use Rage, which is generated by dealing and receiving damage. It constantly decreases, so the best way to keep it up is with an endless stream of carnage. Monks use Spirit, Demon Hunters use dual sources of Hatred & Discipline, Wizards use Arcane Power and Witch Doctors retain the standard Mana. The game regulates these in clever ways, with most classes having a source generator attack, and then a skill which uses your source of power or energy. For example, the Monk generates spirit with a string of attacks and then can use the spirit generated to unleash a different skill, such as a burst of energy or speed or healing. For the magic casters it's more straight up. Wizards arcane power is simply quickly regenerating and every spell uses a certain amount per second or per cast. She also has "signature spells" which require no power to use. Overall, this means every class not only plays differently due to skills, but also in how you attack or what powers your skills. There's no more spamming potions as each potion quaffed has a 30 second cool down.
As you can tell classes are already a big change from Diablo 2. This is further changed by the removal of skill trees and player choice upon leveling up. That's right, no more stat or skill points. Stat points are automatically assigned, and as you level up you simply "unlock" new attacks, skills and passive abilities to use. The way this works is you can only have so many active skills to use at once. Starting out with two slots you unlock a new skill slot every 6 levels. Currently ( and keep in mind this is beta and may change) changing to a new skill puts on a 30 second cool down before you can use it. You also can't swap skills that are on cool down, which forces you to create certain load outs or be more strategic. While somewhat annoying at first I quickly got used to it. With the Wizard I could have 4 active abilities by level 12, meaning I could use my spectral blades, disintegration beam (as awesome as it sounds,) protect myself with some frost armor and cast arcane orbs with a dismissive wave of my hand. There are some very fun skills on display, such as the Demon Hunter's rapid fire and bola shots, or the Witch Doctors crazy array of spells which involve zombie dogs, exploding poison frogs, jars of spiders and other funkiness.
I could be wrong on this, but I'm also pretty sure that in Diablo 2 skills simply did a certain amount or range of damage. In Diablo 3 everything is based off a percentage of your weapon damage. This means the more damage potential for your currently equipped weapon, the better your skills and spells will be. And yes, this meant that at one point in the beta my wizard was running around with a two handed axe as opposed to a wand and spell book simply because it did more spell damage. Overall, the skill system is fun and works well. My only gripe with it is that it removes the fun and excitement of leveling up and getting a skill point to use. It's *yours* to choose to put, and I loved pouring over the various skill trees and deciding what new skill or ability I wanted in Diablo 2. D3's system removes that excitement and I don't like that, but I doubt it's going to change. It works and it's undeniably fun regardless, it's just a change, and as everyone knows, fanboys (and girls) don't like change.
There are a number of other small changes throughout the game. There's an annoying little text box in the bottom left of the screen that never seems to go away and makes me feel like I'm in an MMO. The game also holds your hand a lot, which bothered me. Useful for people entirely new to Diablo 3 or RPGs, but anyone who is a fan of Diablo 2 (which, let's face it, is a lot of people) won't need it. It has all these popups and flashing reminders and tips about things that are basic to Diablo and even basic to RPGs. I found myself always closing them and thought it silly they were there. I didn't see an option to turn them off, but maybe I missed it.