As with the bike choice itself, I think what to carry depends on where you are riding and with what intent. I tend towards carrying everything and the kitchen sink, mostly because I don't like to keep swapping things in and out of my camelback every ride. So whether I'm doing a quick 25km after work on trails on the local mountain, or doing a backcountry ride where not being able to fix something might mean a 25+km walk, I tend to have the following in my camelbak:
*2 spare tubes
*multitool with chain breaker (I have and carry the original topeak alien, but most everything could be done with just a few allen keys and a chainbreaker for significantly less weight)
*spare chain quick-links
*tire boot (a couple of 1 inch square bits of old sidewall)
*traditional patch kit (the kind with rubber cement, not glueless!)
*pump (topeak mountain morph)
*spare derailleur hanger
*couple of energy bars
*lightweight rain/wind jacket
*phone (friend of mine wasn't carrying one when his GF crashed and sustained a major head injury, had to leave her convulsing at the trailside and ride 15 minutes to the trailhead to seek help. Luckily she's OK now but she was in ICU in a coma for weeks and rehab for many months).
*water (I rarely carry less than 1L unless the ride is short and I'm well hydrated before I start)
Whether you need any tyre levers depends on your wheels/tyres. Some multitools (like mine) have tyre levers built into the body, but I rarely find them necessary other than with a brand new tubeless tyre, or really cold hands.
My riding is almost exclusively non-competitive social riding, and even carrying all this c**p I'm usually nearer the front than the back of the group when the gradient turns upwards, so I don't really care about the extra weight (luckily since I'm also mostly riding a 6" travel Giant Reign). Obviously for riding laps on an XC course it would be foolish to carry much at all, let alone if actually racing...
One of my friends is much more minimalist than I, carrying only a spare tube and CO2 inflator all taped to the seatpost for short rides of an hour or so. For slightly longer rides, a single bottle of water in a cage, and a tiny multitool and extra tube are jammed in a wide-mouth bottle in the second cage for rides up to a few hours. For all-day rides he usually carries a camelback with extra spares and water, but still stays relatively minimalist. He probably "borrows" my 'spare' food on 25% of rides!
As for the bottle vs camelbak argument:
Camelbak
*won't fall off
*can drink from it on rough terrain
*can carry more water
*doesn't weigh your frame down (bike is easier to throw around)
*gives your back a little crash protection
*useful for carrying other stuff you probably need anyway
*can feel heavy on your back
*make you hot when working hard (less airflow on your back)
*can swing around and throw your balance off if you don't tighten the straps (e.g when jumping an obstacle)
Bottles
*don't make your back hot and sweaty
*lightweight and simple
*fall off your bike, spill all your water, and litter the trails
*can only be drunk from when you can ride one handed
*fast to swap empty for full at race transitions
*faster to fill
On balance I prefer the camelbak, but if you ride smooth XC trails with long enough straights to get a good drink, bottles are probably a good way to go.