Open the levels 3 wide when its empty. Tighten it down after the first person joins. Players closer to the same level are more successful and happier. Don't be concerned with putting the quest in it. Put all the info in the description, for ex. E 4 BB lvl 5s. When someone joins you can tell them what quest you are in and it makes it less of a hassle to constantly update it when you start a new quest.
Don't be afraid to discriminate. Have standards. If a player won't communicate or cooperate dump them. It's your lfm do whatever you want. But don't be a jerk. Use your ignore list tons it's your best friend. If a player refuses to follow your instructions drop them after the quest is over. And add them to your ignore list.
Putting the quest in the lfm is a good idea especially for quests that take a while and are challenging. Letting your party know your goals, playstyle as far as zerging, optionals and breakables is important as well. Asking for certain classes can be good way of fixing a weak party, but its smarter to switch to easier quests until stronger players join your party rather than waiting around for a balanced party.
Communicate when you are starting, count out loud how many are waiting at the quest entrance to start and let everyone in the party know. This is a simple yet critical item that most people overlook.
Don't take things personally. Manage your expectations. A larger group is more chaotic and many times less fun then a smaller one. If you find one or two great players don't be afraid of closing the lfm because often less is more.
Regarding healing, trapping, just use common sense. It's not rocket science. A trapper makes a great core to a group. Have a cleric hireling if you need it.
Mics are great because you can communicate easily and you should give special preference to mic users.
Failing quests happens and your job as a leader is to encourage success, and pinpoint what went wrong. You should take responsibility for quest failing because you should be aware of the strengths and weaknesses of your party. If you don't know the quest put ¨need a guide" in the lfm and make sure at least one person in the party is familiar with the quest before you commit to it.
Click the option to turn off public grouping (its on the quest entry dialog). If it's set then people can auto join your group and that is bad. You will still get the pop up for a public lfm when someone in your party enters the quest first. Make sure you decline every time. Regarding the hover over quest difficulty setting, I never use it. Just put the difficulty in your description.
Follow these simple rules and you will never lack a group. Have fun and good luck.