Champions Online has some interesting twists on the base mechanics. The basic resource is Energy, which is a cyclical resource much like Rage, Energy, and Runic Power are in WoW. A character starts with a basic auto-attack ability--that differs based on what type of hero you are--which builds energy. Other abilities pause the auto-attack and spend energy. So the basic game-play is to build energy through auto-attack and then spend it on more powerful abilities. This is a very fast cycle, as building the energy meter to full takes 5 or so seconds.
There are a couple of other twists. Energy does not start at full or empty. Rather, there is an "equilibrium point" somewhere in the middle, which marked on a character's energy meter. Energy levels return to the equilibrium point if you are not doing anything. So if you end a fight with more energy than your equilibrium, you will slowly lose energy. If you have less, you gain energy. One of the stats, Recovery, directly affects where the equilibrium is. High Recovery means you start fights with full energy, like Rogue Energy in WoW, while low Recovery acts more like Rage or Runic Power.
Champions has 8 stats: Strength, Dexterity, Ego, Endurance, Presence, Constitution, Recovery, Intelligence.
The stats are fairly straightforward in what they affect. Strength boosts melee damage, Dexterity increases your chance to crit, Constitution increases your health, etc. There are also some more unusual uses of stats. Ego increases the damage multiplier on criticals, Presence modifies your threat (direction depends on what "role" or stance you are in) and powers your heals, and Recovery modifies your equilibrium as stated above.
Now, the element that makes the game work is something called "characteristic focuses". At levels 5 and 13, you pick a stat (so two characteristic focuses in total) and most of your abilities will scale with those stats. This part is really not explained well at all in the game. I learned about it from an Elitist Jerks thread.
What the characteristic focuses allow you to do is take any power and have it work reasonably well. Now, it's best if your stats match up with your abilities. For example, a melee character should have Strength as a focus, because she would get extra damage from Strength and from the focus. However, that melee character could also take a healing power, and it would still scale with her Strength. It wouldn't be as good as a character dedicated to healing who took a focus in Presence would be, but it would be reasonable for soloing or emergencies. It's not like trying to heal in WoW melee DPS gear would be.
Any character can take any power. Powers are arranged into thematic frameworks, like Fire, Ice, Telepathy, Claws, etc. However, you can dip into any framework and mix and match as you please. Each framework is arranged into Tiers. Higher-tier powers can be taken if you have X powers from that framework, or X+Y powers from any framework. For example, you can take a higher-level Fire power if you have 5 Fire powers already, or maybe 8 powers from any framework.
Between characteristic stats and power frameworks, the game encourages you stick with one style for your hero. However, you can easily dip into other frameworks. Indeed, there are a couple points where you must dip, and sometimes it is not obvious at all to a new player that you should. I'll explain more thoroughly further down.
There are also three special types of powers: travel powers, block powers, and passive powers. At level 5, you get a travel power such as flight or super-speed. You can have the power on all the time, for a very minor increase in ability costs. The only thing I note is that Champions is a very different experience if you cannot fly.
Enemies can do a charged attack, denoted by an Adam West Batman-style "Pow" icon above their heads. When this happens, you hold shift to block the attack, or you take massive damage. You can also upgrade the basic block to more powerful blocks later on.
The game has four "stances" or roles. Guardian for soloing, Avenger for DPS, Sentinel for tanking, and Protector for support. Each stance has a slot for a passive power. Avenger wants an offensive passive, Sentinel wants a defensive passive, etc. Your character can have multiple builds prepared with different gear and power loadouts so you can switch between playstyles easily.
Finally, you also get points which you can use to improve powers. You can make powers do more damage/healing, or add additional abilities to the power. For example, add a knockback, or hit a second target etc.
Champions Online is set up for a very fast type of game with interesting twists on typical MMO mechanics. The resource cycle is very quick, and enemies tend to have lower health and come in multiples. The game is set up to encourage characters to focus on specific frameworks, but does allow them to mix and match as much as they want.
As well, having characters focus on two specific stats means the gameplay changes depending on the stats selected. For example, my angel focuses on Constitution and Recovery. This means he has a lot of health and starts fights with a lot of energy. My gunslinger on the other hand focuses on Dexterity and Ego. She doesn't have as much health and often needs to wait for a second or two after the fight starts to build up enough energy to use her good powers, but she has a high critical strike rate and her crits hit hard.
However, the game does not really convey some important information to the player. The whole notion of characteristic focuses is very vague. As well, not every framework contains passive powers. For example, there are only five or so defensive passives, so you might not have one in your initial framework.
This is actually a big problem, because in my view it is vital to take a defensive passive at level 5 or 8. The game is very harsh if you don't, because you will die all the time. You may not even know that you made a mistake, because the defensive passive wasn't even in your framework, and you might not know to look to another framework to get one.
So that's my recommendation for new characters. Take a defensive passive power and an AoE attack as early as you can (by level 8 at the most). Soloing becomes much easier if you do that, and may be excessively hard if you don't. As a personal example, I made two Psions. The first took an offensive buff and a healing power, and I promptly got rocked every time I tried to fight something. The second took a defensive power from the Force framework and an AoE attack, and is rampaging through content that the first could not survive.
I'll post more about other aspects about the game. I do like the mechanical underpinnings of Champions. It seems to allow a wide variety of playstyles to be valid without forcing you into only one style of play. However, I think the game doesn't quite communicate the rules, and thus optimal behavior, properly to new players.