The C gates area = claustrophobic! The space there is narrow and crammed with people during busy flight periods. The only benefit is that the tram goes there..
Terminal 3 is a little confusing to get to if you're picking someone up. I think they need more signage. It has a separate parking area. As for the other terminals, I think the airport is in dire need of a refurbishment. Update the carpets please! I do like that short-term parking is free if you leave within 15 minutes.
The security line can be disorganized when it's busy, I think McCarran tops my longest wait to get screened out of all the airports I've been to.
Free Wi-fi is nice.