I have a grudge against the d20 system. I hate it. I could complain about it all day. I hate it so much that I really can't base this article on it, because I would rant forever.
Restricting myself to one reason I hate it: because hundreds of books have come out for the d20 system. Nothing unifies these books. There is no underlying world, no underlying theme. They're just a bunch of books that have "d20" stamped on the corner.
Because of this, they are compromised. Their promise of a fun and interesting setting is stomped on because they are required to use specific stats, specific conflict resolution methods, specific skills, progressions, dice...
It doesn't make a lick of sense to have a wide range of numeric stats and skills in a game about myth. Fundamentally, mythic adventures are about heroes with superhuman capabilities and flaws. They don't get "12 HP" or a con of "10". They are "stronger than ten mortal men but a temper to match" or "the wisest woman in the land, beware her fell magic!"
The idea of carefully listing out stats and skills is a throwback to the era of wargaming. You can get plenty of complexity and simulationist satisfaction without such crap, and the fact that d20 is basically D&D with all the funny-shaped dice replaced with dodecahedrons makes it fundamentally unsuited for most kinds of adventures.
I see this problem with all "shared" systems, including things like GURPS, Hero, Rifts, D&D, and even the almost unknown ones like d6 and Silhouette. Not to mention the huge number of games which basically use the same system, but aren't affiliated. If it's got stats running from 1-20 and a hundred skills, it's really just D&D.
These systems are often good at a particular kind of story. But they do not limit themselves to that kind of adventure, and instead publish add-ons for adventures that make no sense at all. Like D&D's "legendary" level adventures. D&D's system is fundamentally unsuited for legendary-level characters. The GM basically has to work around the system and hope his players aren't smart enough to pry at the cracks.
This is why I'm pushing for people to create their own system when they create their own setting. You have a great idea about the players all playing clerics? Great! Just don't put it in D&D, because their rules are painfully shattered when it comes to that kind of focus. Build your own system, a system with rules that fundamentally support the various kinds of faiths and capabilities of clerics of various gods.
I realize that I'll probably catch some flak. Someone will probably say, "It's not D&D, it's AD&D 3rd ed, which is completely different". Actually, that's the point: an RPG system is not like Street Fighter. You shouldn't have D&D, AD&D, AD&D Alpha Turbo Edition, etc. You should make a game that does precisely what you want it to do.
If your system doesn't support eight clerics in the party, you shouldn't stretch the system. You should create a new system for an "eight clerics" adventure.