I have discovered the secret to running a good 5-man, be it PuG, guild, whatever! Have me in it.
Sorry, I couldn't resist myself. :)
Seriously though, it's what I call the key players. If you have good key players, no matter what else you fill in the middle with, as long as they aren't complete dummies, you'll have a good run.
The good key players ... you can probably guess what ones they are.
Now, this idea is probably formulated because that's the group that Boon, Owaru and I typically run in. One a tank. One a healer. One a DPS/CC. Is some of it playability? Sure, we kickass, take names and look pretty. But part of it is very simply that with a decent tank, a decent healer and a decent DPS/CC in the group, it's hard to screw up enough that you can't succeed (unless you're in way over your head).
Everyone's played with a bad tank. The healer pulls aggro by simple healing on a single target pull. They take forever to pull. They take forever to mark. They take forever and a day to build threat. They lose threat to a sneeze. They have craptacular armor/health and/or don't know how to hold aggro for their class.
Bad healers are also really tough to succeed with. They don't heal. They don't heal on time or with a heal that does any good. They run out of mana 2 seconds into a fight. People dying left and right! They wait too long between heal casts. They wait too long to START a heal on someone. You know how it is.
If you don't have a good DPS/CC class, at least one good one, then nothing gets accomplished. The tank has to hold aggro too long and non-repeatable CC wakes up (sap) or the healer runs out of mana because things aren't dying, or the CC keep running amok instead of being put back to sleep/sheep/etc causing the healer to 1: take hits and 2: keep himself and the dps and the tank all alive, which makes them run out of mana faster. Or almost worse, the DPS is the 'MUST BE T3H AW3S0M3!' and feels if they aren't top of the DPS chart, regardless of aggro, then they are the suxxors. (My elite speech isn't up to date, sorry if I'm using the wrong terminology!)
If any one of these three things are missing, it makes it really tough to succeed. The more key of the key players are the tank and healer. If one of those is missing, it's going to be tough to succeed. The DPS is a nice tie in, lending a strong support system to the tank and healer, but it is possible to succeed with only a good tank and a good healer.
HOWEVER... if you have all these three things with decent people in those places... all of a sudden, you have a recipe for success.
The tank, being a good tank, pulls appropriately, marks timely, keeps things moving and well paced. Keeps mobs off of the DPS and the healer. Keeps everything and it's brother hitting just him (or her!)
The healer, being a good healer, gives timely heals, is ready with a shield or a HoT or a something else if the mob turns its attention off of the tank. Has sufficient mana for a given 'regular' pull in said instance.
The DPS, being a good DPSer, does as much damage as they can without pulling off of the tank. They CC correctly and on time and continuously. Good consistent DPS and CC.
The other positions in the party are important, but not as important as the three key players. You can survive just about any five man with one really good tank, a really good healer and a really good DPS.
But without those three key spots filled with good people, you can't carry poor DPS/CC. With them in place, any DPS/CC, good or not so good, can be helped along to success.
In relation to this, people accuse Boon, Owaru and myself of having a clique. This is not true. We run with one another more frequently and preferentially because we know one another. We've been partying together for a really long time. We know what each other is likely to do in a given situation. Owaru can trust me to re-CC and pull aggro. Boon can trust me to DeathCoil him. We trust one another. So we play together more frequently and preferentially because we're good together. And together we typically have success. Why should I look for another tank when I have Owaru? Why should I look for another healer when I have Boon? In any incarnation of roles, Owaru and Boon are two sides of my party-triangle.
<3 you guys! (Boon slightly more than Owaru.) On a downnote... helping those two poor struggling DPS/CC to success without them having to do much of anything just perpetuates poor DPS/CC in the higher levels, dungeons, heroics and raids. How many times does someone get carried along in Kara or a heroic because 'we need the spot filled'?