Here are some of my thoughts on converting a 3E campaign to C&C. (I will be doing this myself in the near future.)
Overall
As Henry said, since C&C draws on the same tradition as 3E, many of the 'core concepts' remain the same. C&C Wizards are like 3E Wizards, C&C Elves are like 3E Elves, and so forth. Any campaign setting that you can use with 3E (Greyhawk, FR, homebrew, etc.) can be used with C&C. A setting that introduces and/or relies on a lot of new 'crunch' (e.g. Eberron, with its new races and classes, etc.) could probably be converted, albeit with a little work.
Monsters
Contrary to what you suggest, material from the Monster Manuals (and other 3E monster books) actually would be the
easiest stuff to convert to C&C. Most of the conversion could be done 'on the fly'.
The main thing that you would have to do is determine what the monster's "good abilities" are -- "Mental", "Physical", of
both. This should be easy just by looking at the creature's skills (if any), saving throws (high Will would be "mental"; high fortitude would be "physical"), feats/classes (if any), and so forth. Then just make an overall judgement as to whether the monster's "good abilities" are mental, physical, or both.
Armor class, most spells, most special abilities, and so forth, could be used 'as is' in C&C. Some things that depend on AoOs or skills could either be ignored or hand-waved, as you see fit.
The overall 'power level' in C&C is slightly lower than the overall 'power level' in 3E, so you may have to keep this in mind when using a 3E monster (e.g. a monster that might be appropriate for a 4th level 3E party might be appropriate for a 6th level C&C party). Alternatively, you could weaken the 3E party a bit (fewer HD, etc.).
Feats, Skills, Spells, etc.
Other 3E material will be more difficult to use. There is no 'feat system' in C&C, though I suppose you could add one if you wanted. (You would need to be careful to make sure that the feat system benefits all characters equally, as they are already 'balanced' in the core rules. Hence fighters should not get more feats than other characters, as feats are not necessary to make the fighter 'equal' to other classes.)
Alternatively, you can let players 'fine-tune' their characters by 'trading' a class ability for an equivalent feat. This would require a judgment call on your part, but I don't think it would be unbalancing, say, to let fighters 'trade' their 'combat dominance' ability for 'two weapon fighting'. The same thing could be done with some class abilities and skills. For example, if you wanted to play an 'acrobatic' kind of thief, you could substitute the rogue's 'pick pocket' ability for the 'balance' and 'tumble' skills.
An optional skills system will be available at some point (IIRC). However, adding one should be relatively easy, so long as it benefits all classes equally. (Again, all classes are already 'balanced', so a rogue does not need more skill points in order to be 'equal' to other classes.) Something along the lines of the RC skill system would work well IMO.
Alternatively, you could use the 3E skills, after removing all the skills that correspond to specific 'class abilities' (disarm traps, etc.). The remaining skills could be chosen by all classes. Giving them 2+Int bonus "skill points" to allocate each level would work fine, I think.
Spells are easy: since the C&C system draws on the SRD for most of its spell descriptions, using additional spells found in 3E books should be fine.
Classes
There are C&C equivalents for all the standard 3E classes -- plus a few extras (Knight class). The classes differ in some respects: the paladin, ranger, and bard classes are not spellcasting classes, for example. So if you prefer your paladins to have spells, some tweaking will be necessary. (IMO I like the fact that the spellcasting classes are limited in C&C -- I have always thought that spellcasting was too ubiquitous in D&D.)
Those are my thoughts for now.
