By and large, I don't care for class systems most times, and I'm on the fence over levels depending on how they're implemented. I make an exception for D&D because in D&D, they do prove to be rather fun as a game mechanic, and also because the world of D&D is essentially defined by those classes.
First I am a long time GURPS it has been and continues to be my primary system. With that being said I have a lot of respect for the D20 system. From my point of view if D20 existed in 1986, I would have never switched to Fantasy HERO and later GURPS.
Why? Because one of the main reason for switching systems it being able to customize your characters. In the beginning most did it through acquiring items but that had its own problems. A lot of players wanted to customize their character's capability.
D20 does that well through the feat and skill subsystems. No two fighters have to be the same (clerics, wizards, etc, etc). The multiclassing rules is just icing on the cake and allows for truly unique combinations.
When I got 3rd Edition, I both played it and GMed. To me, as long time GURPS DM, it looked like a GURPS lite. The 20 levels were a chunky way of allocating "points", attributes were attributes, skills were skills, and feats were advantages, and no disadvantages. When I DMed a game for a bunch of old time 2nd Edition players I opened their eyes as I ran it pretty much like a GURPS game with lots of skill checks and using their feats (advantages). My NPCs were nearly always a combination of classes including NPC classes.
In fact I briefly considered permanently switching. The main reason is that it was a pain the ass to make GURPS NPCs and creatures and D20 was considerably easier.
But in the end I didn't make the switch. I didn't feel like converting the decade long mass of support material I developed for the Wilderlands. GURPS 3rd Edition started publishing templates which eased my NPC creation problems. Finally when I played through D20 I didn't like the rate of progression (too fast). So I reverted to GURPS. But D20 is a fine system and would use it under the right circumstances.
There is nothing about the Class, Attribute, Feat, and Skills system that ties it to D&D and High Fantasy. It is eminently customizable in my opinion. Obviously the D20 SRD is for D&D High Fantasy. But the basic principles are sound and I feel been successfully applied to many different genres.
Classes are just a shorthand for a template to use to buy X number of Skills, Feats, and ability increases. Really no different then using GURPS templates except that few GURPS Templates have different levels of experiences while Class/Prestige Classes come in 5,10, 20 levels of experience.
From playing D&D for 20+ years the main appeal between D&D and the other rpgs was that with other RPGs your character was represented by bag of skills and abilities that was customizable. D&D in contrasts everything depended on the class you took. D20 is definitely in the category of "other" rpgs with your character being represented mostly by a bag of skills and feats. (with a few class abilities thrown in).
In contrast Castles and Crusade is more like D&D with actual abilities being firmly tied to the class. Of course you have the siege mechanic which makes it more flexibile the original.
Rob Conley