To expand on what Jahydin said.Some tips!:
Apologies if I've already mentioned these to you before...
1. Ability Checks are only used for resolving something interesting! The default assumption is a competent adventurer attempting a check has a 50/50 chance of success, so most PC actions should be resolved with common sense.
2. The most common houserule (that actually alters the game) I've seen is changing the Challenge Bases (12/18) to fit the CK's style of game. 12/15 or 10/15 are popular choices. I use the Tertiary Optional rule in the CKG which allows players to select 3 Primes, 2 Secondaries, and 1 Tertiary Attributes, so 12/15/18. I think this makes PCs much more well rounded.
3. Use one Challenge Base: 18. Primes give a +6. Mechanically this is exactly the same, but so much easier to use.
4. Check out the rules for Class and a Half leveling! Essentially, it allows players to gain levels in two classes (a Full class and a Half class) at the expense of needing extra Exp. The rules are so simple and clever, you'll wonder why other systems don't use it.
5. If you want Saving Throws similar to AD&D, just lower the Challenge Base by 1 every other Level.
My understanding is that C&C is still in its "first edition" and probably always will be, so whenever someone comes up with a cool new way of doing something, it goes in as an "optional rule".
So if you prefer some of the optional rules (for example, I couldn't imagine playing without using number 3, above) then don't feel they are in any way less "official" or "it can't be a better rule otherwise it would be the default".