When playing WOW you first level yourself up by bashing monsters in the head for 85 levels. This used to be incredibly simple and straightforward - you run up to the monster and beat on it until it dies, then you go to the next one. Eventually you find that your level is at the maximum and the next step is to start raiding, which is destroying difficult challenges with many people. The challenges of raiding are substantial compared to soloing, and in particular it requires all kinds of skills that you simply had no place to learn while soloing. The classic example is getting out of the fire. Bosses in raids put fire on the ground and you have to not be in it, because it will burn you very badly. Sometimes the fire is actually poison, or frost, or whatever, but it is bad. The trouble was that when soloing this never, ever occurred and a new raider would find themselves trying to master a skill they had zero opportunity to learn on their way up. This was a flaw in the game's implementation, that skills that are considered baseline and necessary in the endgame did not appear at all in the early game.
Cataclysm has changed all this, and done it beautifully. Previously when a monster in WOW was supposed to be hard to solo it would simply be given a few more hitpoints and it would do massively more damage. It was much harder certainly but generally the tactics required were no different. In Cataclysm there are all kinds of solo encounters with monsters that have immense hitpoint totals but who do very little damage, thus the fight can continue a long time, just like raid encounters. These encounters also have mechanics exactly like raids in that bosses put fire on the ground or do incredibly destructive attacks that can be entirely avoided with proper reactions. People like myself who have raided forever will mostly find these mechanics very simple but they do still lend an air of grandeur to a solo encounter that was sorely lacking before. The screen will give you instructions on how you can avoid these terrible attacks
"Overload Zingband is casting a Shadowbolt Volley! Find someplace to hide!"
If you hide behind a pillar or somesuch the Shadowbolt Volley does nothing, but if you just stand there and take it you will die. Just like in a raid you need to be constantly attacking the enemy and occasionally dodging their devastating moves. It is wonderful!
The combination of giving a little basic training to newbies and providing a bit of fun, variety and epic feel for a veteran is a great one and this is one of the great success stories of this new expansion. Now if you level up to max and get started raiding you will definitely have experience in dealing with all kinds of raid challenges - interruption of casting, movement, line of sight strategy and many others. There are a lot of things that cataclysm did right but to my mind this is the biggest - make the transition from soloing to raiding smoother and make soloing feel much more meaningful in the process.