Gear, of course, makes a big difference. If your shaman has a few epics on him, chances are he can take a few hits before the tank has a chance to grab a mob off. And a group with tons of DPS can do really well at tearing down a mob as they pull them-- I've played in groups where we never once bothered with sheeping. We just had so much DPS that when we pulled something off of the tank, we killed it before it could hit anyone.
Not to mention that even if you have a group without CC, there are plenty of abilities that you can use as CC. During my Heroic Slave Pens run this past weekend, we had me (a resto shaman), a prot warrior and a holy priest, a moonkin druid, and a warlock. Whoops, no sheeping or traps, right? Nope, but we did have cyclone, chain fear, my elemental pets, and mind control. Ok, well, my elemental pets did drop pretty quick (it was heroic, after all), but incredibly (even though we were a little undergeared for the instance), we made it far enough to pick up two Badges each. Yeah, the mind controller gave us a little trouble, but it was getting late anyway, so we called it.
CC helps, and you do kind of need the basics (tank, healer, and DPS, although it's true that even with those, you can make some subs that don't seem possible), but the truth is that the limit of your group is the limit of the players in it. All of the classes have some ability to keep mobs out of play (or just keep them busy for a little bit until you're ready to fight them). It's just up to players to think of creative ways to do so.