I went through various phases of Comic Book fandom, so superheroes are a big part of my worldview. No, I don't wear superman underwear anymore, but I've caught all the major Superhero moves to hit the big screens (not all ON the big screen, there were a few I waited for DVD for). And as a fan of role-playing, I've always wanted to play out some of the grand comic plots with a pile of dice in front of me.
Most of my gaming experience has been through Dungeons and Dragons (primarily as a player, though I dabbled a bit with DMing a play-by-email game), so when I first heard about Mutants and Masterminds (back in 1st edition) I was extremely excited. I picked up both 1st and 2nd edition, but never had a chance to actually play them. It wasn't till 3rd edition that I decided,if I wanted to play a Mutants and Masterminds game, I was going to have to GM it myself. So, when my group was between adventures and no one seemed particularly interested in being the next GM, I bit back my fears and volunteered to run a game set in Freedom City.
Response from my players was enthusiastic, and I set up an adventure based on a sample adventure from 1st edition. The Freedom League are acting strange, committing crimes and it's up to the PCs to find out what's happening. Everyone had a fun time, and my first in-person GMing experience was a success. Thank you Green Ronin, especially everyone on the Mutants and Masterminds team.
Now, discussing the rules themselves. As a Dungeons and Dragons player, still stuck in the era of 3rd edition, the D&D heritage is still obvious in M&M, though lots of bits have been filed off. Recognizing that the actual ability scores are almost never used in D&D and the ability bonuses are important, M&M 3 makes the bonuses the actual scores. Your average man on the street has a Strength of 0. A body builder might have a Strength of 4 (equivalent to D&D strength 18). Skills have been consolidated down (Stealth encompasses Hide and Move Silently, Perception encompasses Spot and Listen). Attacks of Opportunity, flanking and a lot of tactical rules are gone.
The biggest difference you might notice is the replacement of the Hit Point mechanic with the Toughness Save. Rather than have the attacker roll dice to determine hit points of damage, the defender rolls his toughness save against a DC set by the strength of the attack. Failure applies a cumulative -1 to all damage saves in the future. Failure by at least 5, or 10 apply additional effects and finally failure by 15 results in a knockout. This mechanic is a little difficult to get used to, but it works pretty well. We had a few enemies that just would not finally go down for the count. I feel it added some suspense, with -5 to your toughness save, you never know whether the next hit will be the one to take you down. With 5 hit points left, you're pretty sure that it will be.
Other powers, like Mind Control, Sleep, Holds are all handled by a versatile power called Affliction. Like Damage, these have various levels of effect based on how much the defender fails there save by. This makes a nice comparison with damage, so neither knocking an opponent out nor putting them to sleep seems like a cheap way to end the combat.
Mutant and Masterminds, like many other supers games, uses a point-buy system. This allows large amounts of freedom in character builds. Math is necessary, but just addition and simple integer multiplication, nothing as complex as Champions required. I used an excel spreadsheet for my first few characters, but then found an excellent product called Hero Lab that has great Mutant and Masterminds support. More important than point-balancing though, is the concept of Power Level. Without power level, a character could simply throw all their points into Strength and Close Combat and do 50 damage with +50 to hit. To allow players to have versatile powers, without being hopelessly outclassed by these heavy-hitters, Power Level limits the sum of the attack bonus and the attack rank to two times Power Level. At the default Power Level of 10, a hero with Strength 10 can only have +10 to hit. Similar caps apply for defense bonus and toughness.
One result of Power Level, is it makes improvising villains extremely easy. You can pick a power level, and just pick your villain's attack bonus, attack rank and defense so that everything hits its caps. There is enough flexibility in the system that you can be assured that there's some way to legally reach those values, and since NPCs don't have to worry about how many points they're built on, it is often not important exactly how they reach those values (Especially if Captain Awesome is just going to punch them right into the jail cell).
My favorite advantage of this game over something like Champions is the lack of endurance. There's no keeping track of how much "endurance" or "mana" or "power" you have left after each attack. Captain Awesome can punch Unstoppable Wall at full strength all day if he has to. This not only removes an annoying bit of bookkeeping, but it fits the comics better in my opinion.
There is a lot of hand-waving involved in the game. I personally feel that Affliction is overused, and some effects would benefit from specific rules instead. There are some advantages and power modifiers that can quickly break power level and grant characters attacks much stronger than expected. But, overall the game is fun if you and your players can get into the superhero mindset.
For DC fans, there"s DC Adventures. This is the same rule-set as M&M 3rd edition (and was actually released first), but it contains artwork from DC comics, and stats for a range of heroes and villains. The supplement Heroes and Villains Volume 1 contains over 280 (by their count) DC comic characters, from Abra Kadabra to Kobra. Volume II is in progress to cover the rest of the alphabet. Even if you're not playing in DC, the book provides a ton of ideas and characters you can slap a new name on and run with (or leave as is if you're players won't complain about Deathstroke or Nightwing showing up in Freedom City)