Four-Color to Fantasy, published by Natural 20 Press, was designed with two goals.
1. To create a rules supplement that would add super-powers to any existing D20 game, be that Spycraft, D&D, D20 Modern, or DragonStar.
2. To be effect-based (and not appearance-based) in regard to the powers.
A third, less important goal was to make sure that with just the core D&D rulebooks and FCTF, you could play in a modern-day game, as Horacio has been doing with his
Golden Apple Rescue Squad storyhour.
Since practically all D20 games so far use levels, we decided to simply introduce a new class that could provide super powers the same way that a sorcerer gains spells. Called the Hero, this class goes from 1st level to 20th level like all the core classes. All the game master needs to do is decide a power level for the group, and you design your character with the appropriate number of Hero levels.
Of course, just because the class uses levels doesn't mean that you can automatically just level-up in it. The Hero class is much like a powerful race that provides a level adjustment. The default assumption is that the GM will just pick a level, and everyone will stay at that level, never increasing their superpowers (though they can still level up in other classes). However, if the GM lets you, you can go up levels in Hero, assuming your explanation for your power increase is approved by the GM.
We also provide a small appendix for modern-day skills and technology, and present a wholly skill-based class, the Specialist (imagine a rogue with less combat ability and no sneak attack, but with more ways to use their skills in cunning ways).
Basically, by using levels, and making sure the Hero class is balanced with the core d20 classes, it ensures that game masters are able to properly gauge how difficult an encounter will be for the party. Also, it means that you can use most other WotC and 3rd-party rules supplements in your supers game.
Why would you want to use D&D rules (which are supposedly made for fantasy) in your modern superhero game, you ask? Well, consider the following character types. We'll assume your top-name heroes are 20th level or so (though some may go higher).
Spiderman: He starts off as a 1st level Specialist (photojournalist-scientist). Then, when he gets bitten by the spider, he gains 10 levels of Hero. Occasionally in the comics his super powers will get a little stronger, but not by much. However, as he does his superhero stuff, he slowly gains levels as both Specialist and Fighter. Eventually, he might take a level or two of the Gadgeteer prestige class from FCTF.
His powers would consist of super strength, super agility, flight (with a few special restrictions to simulate web-swinging), danger sense, and entangle (to represent his webs). As he gained levels, he could use his new Hero Points to purchase some of the tricks he learned, like using webbing to make shields, or gaining gadgets like tracking devices.
Eventually, at around the current day, Peter Parker would probably be a Specialist 3/Fighter 4/Gadgeteer 2/Hero 11.
Superman: Everyone knows his powers, and he'd probably have epic levels of Hero, plus a few in Specialist (journalist), and maybe a few in Fighter in his later years. Making high-level foes to challenge him in one-on-one combat would be tough, but thanks to the Monster Manual and the Epic-Level Handbook, you have ready-made alien invaders, ancient evils, and huge monsters.
Batman: When D20 Modern comes out, you'll want to give him about 3 levels each of Strong, Fast, Tough, Smart, Dedicated, and Charismatic Hero. Or, if you don't want to buy D20 Modern, just make him a Specialist (entrepeneur) 2/Fighter 6/Rogue 3/Hero 3, and give him 3 levels each of the FCTF prestige classes Detective and Menacing Vigilante. You may ask why Batman has Hero levels when he doesn't have any super powers. However, he does have mild super strength and mild super agility. It's nothing compared to Superman, but he probably didn't roll all those 18s.
As other examples, Dr. Strange probably has a few levels of Mongoose's Demonologist class (or he might just be a sorcerer), DareDevil might benefit from some combat feats in Path of the Sword, and Fathom could definitely use the underwater combat rules in the various D20 ocean and sea books. Heck, half of the people from Image are practically fantasy characters anyway, and we know danged well that Lara Croft could definitely make use of the Dungeon Delver in Song & Silence.
FCTF can be used with ease with pretty much any d20 rule system you want, and it doesn't try to codify every superhero into a set of classes like Aquatic Warrior or Telepath. The power system is flexible without focusing on minutiae, and for the few exotic powers that we might have missed, creating new ones is easy. Plus, not to be petty, but visuals do matter to some people, and I'm confident that Four-Color to Fantasy has, overall, the highest quality visual layout of all the books presented so far.
However, I do suggest that you also take a look at Mutants & Masterminds (from Green Ronin, out in a few months). It's not as compatible with core D20 rules as FCTF, but it goes for a much more streamlined type of play, with less dice rolling that might suit your tastes.
Thanks again, Morrus!), I can testify that everything RW said is true. 
