There is a bare minimum for Heroics. You should at the very least try to obtain these minimums before you step into them and drag you fellow puggers down. Obviously guild and friend runs may give a little, but not too much. You dont want to be "That Guy".
"That Guy" is the guy (or girl) who comes to your pug run with mid 70 greens with no gems or enchants. They make everyone else work that much harder and generally speak mostly in leet speak.
Here are some minimum guidelines for a few different roles in Heroics.
Tank: The very bare minumum a pug tank should be running is 21-23k health and 535 defense. At this point, being below defense cap is not really an option. Heroic mobs hit somewhat hard, and getting crit a few times on a group pull will quickly kill a tank with so few hitpoints. These numbers are recommended for the easy Heroics. Some of the harder ones will have a bit steeper requirements. This threshold should be incredibly easy to get. If you have been doing instances on the way up and picking up the gear, then you should be in pretty good shape once you hit 80.
Once you hit 80, there are several very nice crafted pieces for you to pick up. The three Titansteel tanking pieces (Shield, head, and boots) are a great start for a Warrior tank. Replace the shield with the two handed mace for a DK. As soon as you hit 80, start grinding the Wyrmrest Rep (if you are a plate tank). They have very useful rewards that will help you achieve your beginning heroic goals.
I stepped into Heroics at 23k health. Do not be afraid to ask for a little bit of CC. When I first started hitting the Heroics, I made good use of my buddy's repentance spell. Do what you have to do to make it smooth.
You dont want to be the guy I kicked from group the other day. He tried to tank a Heroic with 18k health and 480 defense. I had about that much at level 70. That is absolutely not acceptable for a Heroic. 18K health is acceptable for DPS....not a tank.
Healers: Healers just starting Heroics should have somewhere in the area of 1200 spellpower minimum. This is not incredibly difficult to obtain. My level 70 Warlock has over 1000 as does my Paladin in his level 70 gear. It may be possible to do the Heroics in under the minimum, but you will be having a very rough time of it and probably run out of mana somewhat quickly.
Crafter items are the easiest way to get to this minimum. When I hit 80 on my Druid, I had the leather healing pants and boots as well as the cloth healing chest and gloves waiting for me. I had the Titansteel Guardian made shortly after that. These items are easy to obtain if you have a bit of cash (or the required material. If you pick up these items, make sure to enchant them. They dont have to be best in slot enchants, but they should be useful. Firey is not useful for your weapon 0_o. The easier it is the heal these Heroics, the easier it is for you to obtain new gear, and the less wipes you will have.
Ranged DPS: I believe that most spell casting ranged dps (mage,warlock, boomkin) should have somewhere in the neighborhood of 1000 spellpower, and should be actively working towards the hit cap. These should be pretty easy to obtain since the Ebonweave gear gives nice big chunks of hit on it. If you are a class that uses a decent amount of crit (mages, destro locks, Boomkins), you should probably be around 20 percent to start. With talents and gear this should also be very obtainable.
Hunters are physical DPS and obviously dont want spellpower gear (No, you can not use it for a mend pet set, and yes I have actually heard this before). You should be starting out at aroun 2k AP, working towards the hit cap, and 20 percent crit. You should also pick up a nice weapon since it is really the single biggest upgrade a Hunter can get to their DPS. I recommend the Nessingwary 4000 since it is craftable. If you dont like the bang bang of the guns, you can always try to run Heroic UK a bunch of times for the bow (since it really is the easiest Heroic).
Melee DPS: Melee DPS should be following the same rules as Hunters pretty much as far as requirements go. Obviously the gear will be different from class to class. Follow the same rules as above for gearing. Make sure you have the questlines completed that you need (aka Hodir and Ebon Blade). Enchant your gear. Profit.
Guilds: I brought up Guilds earlier for a reason. Guilds often help their people gear up. It is one of the perks of being in a guild. They will often bring along overgeared players to help you. You can nix most of the requirements above if your guild is helping you steamroll. The only one I would be wary on is the tanking requirements. Tanks take the brunt of the damage and no matter how good a healer is, they can get gibbed without appropriate stats.
If you go into pugs and have obtained these minimums, you will save yourself from looking like a fool. Getting Outdpsed by the tank is not a fun experience, and should frankly be embarassing. Get your stats, then get your Heroic on.