Well, I tried the demo out...
...I still have no idea why people like Monster Hunter so much. Would it have killed them to make a demo that goes a little further to explain the game?
After reading the description of what went down, I'm so extremely tempted to Dante avatar-quote...probably wisely thinking better of it.
Anyway to answer some of your concerns:
Given that it sounds like you played the 3DS version of the demo, you can control the camera either with the physical D-pad on the left side of the system, or the digital D-pad on the touch-screen, which is there so you can orient the camera with your right thumb while still moving. Alternatively, once a large monster (like the Lagombi in the demo) spots you, you can tap its icon on the touch screen and from then on a tap of the L-button will center the camera on the monster. There is not a pure lock-on that fixes the camera on the monster.
In most quests, your task is to hunt a particular monster or several monsters (by killing or capturing them). Some early quests also send you out into the world to gather particular items or kill off small mobs of monsters, and in the full game these are the quests built to familiarize you with the areas of the game and the basic controls. Speaking of controls, every weapon controls differently although a few things are shared (like L being used for camera centering and item-switching, B for dodging/rolling, and Y to sheathe your weapon and use items). Once your weapon is out, you have X, A, X+A, R, and other such combinations along with the control stick to access whatever moves you have with that weapon. Use the digital manual for a more full listing of the attacks each weapon has and experiment to figure out which are best to use, and when.
Boss monsters do not get health-bars, but will let on certain cues depending on what state they're in. They can be normal, enraged, exhausted, weakened, etc. Repeated blows to the head will cause monsters to flinch and stagger for a second, most monsters have breakable parts on them that will also cause staggers and will increase your chances of getting good loot from clearing the quest. The loot is then used to upgrade your gear, which gives you more power and different abilities to take on the monsters that come next. All the while, your actual skill at playing the game increases, and the more you fight the monsters, the more you learn their attacks and their tendencies, which makes them easier to manage the next time you face them. It's a game about learning, prediction, and precision (and proper teamwork, if you're playing co-op).
As for the demo...even most all of the vets here agree that the demo doesn't do any favors for people who have no prior knowledge of Monster Hunter. There's a lot to the learning curve here, more than you can adequately experience in 20 minutes. My personal assumption, I think that the demo was originally built for the release of 3G in Japan, where MH is already a massive franchise and they assumed that most of the people playing it would be experienced. I said this earlier in the thread and I'll give you the same advice: try the demo out a couple more times, possibly even using the same weapon a few times in a row. Try to pay attention to the monster's attacks, pick out particular openings to strike and then get away before being hit. Aim to cut down how long it takes to fight the monster, and strive to use as few recovery items as possible. It's that sort of pursuit to perfection that keeps monster hunters motivated.