Most people leading a pug don't set it up in hopes of failing. They normally want to have the most efficient run possible. Quick and painless. When they are spamming LFG, you rarely see "LF more fail players for Naxx 10. Must have never cleared past Anub. Green, ungemmed, unenchanted gear a plus, PST!"
As a raid leader, there are several things you can do to help insure that the run will be at least somewhat successful. The first one is very simple. As the player how much of Naxx they have done ON THAT CHARACTER.
Some people will tell you that they have fully cleared the instance. What they dont tell you is that they did it on their main, and this is their scrubby fresh level 80 hunter with 120 hit and doing 1100 dps.
This leads to the next part......ARMORY is your friend. You can find a lot of information out on Armory. You can check out their gear and raiding achivements, which at least gives you something to go on.
Gear should never be a lone determining factor in letting someone into your run. That said, you should not ignore it either. If someone is wearing 75 percent greens, it shows that they made absolutely no effort to gear up before hitting the raids. They would rather have free epics spoon fed to them.
Green gear should not be an automatic no. There are several green pieces that are pretty decent....they should hopefully have more blue gear or even a few epics (since you can craft so many of them). Gear doesnt make the player, but it will show the amount of effort they put into gearing.
When you are making a raid, you are usually looking for some nice balance. A ten man raid with 7 DKs can certainly work, but is not at all balanced for buffs (or ranged dps for that matter). They say you should bring the player, not the class, but different classes give different buffs. If I am making a raid, I usually like to have a very nice balance including at least one Paladin, Priest, and Shaman. These are not essential, but they have the buffs to make your job that much easier.
When you are putting your group together, if you are planning on using ventrilo, verify that everyone you are inviting has that capability. It is annoying when you are trying to explain something and that one player is not there and does something dumb.
Once you send out all of your invites, make the rules of the run known. Most of them will be wondering about Loot Rules. Get these out of the way up front. Also, let them know if there is going performance rules. This could be as simple as "Anyone doing below 2k dps will be kicked from the raid". You do not have to do this, but it can keep DPS on their toes and keep them from facerolling their way through.
Everything that you are expecting from the participants in your raid should probably be related in the LFM message. "LF 2 dps and 1 healer for Naxx 10. Must be decently geared and know the fights. DPS must be able to do over 2k dps. AFKs will be kept to a minimum. Too many will get you kicked. PST for invite."
If you follow these rules, you may save yourself a few headaches, and will be well on your way to a nice raid.