The biggest "non-old-school" thing about it was the Spell list. It was really just a thinly modified 3e spell list and I found it very wanting.
This was one of the things that I had to modify, when running C&C. I remember being particularly bothered by the Cleric's
sound burst spell. I'm big on the classes being archetypes and having their own niche, and see clerics as being a combination of a fighter and a spellcaster, but "second class" at both (i.e. not as good in melee, not as potent with spells). That kind of spell let the cleric out-do the magic user; it's a better offensive spell than sleep, for example.
I added a bunch of house rules, of course. Many of them made it more like AD&D1e (movement, weapon list, surprise, combat mods).
Yeah, me too. Surprise was a big one. Using the SIEGE engine for surprise had some weird side effects because perception was Wis-based, and Clerics were almost always high-Wis and Prime-Wis, so they were usually the best ambush detectors in the group.
I changed movement, too. I like the way turns and movement work in the old editions; it makes tracking time by movement (during exploration, for example) very simple and intuitive. I also changed movement in combat; C&C's rules were slightly different from what I wanted.
Saving throws were another area where I initially thought the SIEGE engine was cool, but gradually realized I liked the old approach better. SIEGE-based saving throws tie are "resistance-type" affairs based on your stat, your level, and the level of the effect you're saving against. As I mentioned, above, I'm a proponent of classes and archetypes, and I like rules that reinforce (rather than break down) the importance of your class, so the SIEGE-based approach to saving throws didn't end up doing what I liked. Also, I found that having the opposition effect's level always modifying saves caused them to work differently than I wanted. In short, it's not that C&C's save system is bad, but rather that it doesn't model saving throws like I want them to work.
I don't think that the Seige System adds all that much to the party.
My experience is the same. I ended up cutting the SIEGE engine out of surprise and saves, as I mentioned. I also found myself relying on it less and less for handling other actions, as time went on.
Still, C&C was totally serviceable.
I want to reinforce that sentiment, because despite my criticisms, I had a good time with C&C, and it did the job. Also, my criticisms are very much rooted in my preferences and ideas about how the game should model this or that.
I think C&C is a great
via media that stakes out a position between "old school" and "new school" and allows individual DMs to easily pull from the various editions. One C&C game might be very AD&Dish, while another might feel d20-ish, depending on the people involved.