I figure I should pay tribute to another outstanding site, The Egotistical Priest.
Not just priest focus here, lots of general background. My particular favorite is her writeup of a 5-man grouping.
Quite honestly, my previous vent about wanting to find instance groups for pre-70 toons should probably be altered. What I really want is a guild that has leadership who subscribe to Ego's school of thought.
So many PUGs, and even inexperienced guilds, give all these reasons and excuses for why you need such-and-such fancy team makeup, or why you can't survive without 2 rogues, or why its OK for leather-wearing DPS factories to go nuclear with no regard for threat/aggro.
I know we could pwn an instance that's at, or a drop below, our lvl with leather tanks, because the mobs burn down so quickly, that a heal or two is all it takes to survive. Hell, at this point, if one of my guildies is even able to steal aggro from my Teddy, he's lucky. And, if a leather tank feels she's taking too much damage, she just pops in a couple more combo moves and, poof, the mob's dead and its all good.
Now, I can't speak from experience because I haven't been there yet :-( but excellent posts like Ego's sure seem to indicate that the holy trinity of tank/healer/dps is something every newbie should be taking to heart.