"1) Is there anything that all of you, in your numerous collective years of experience, feel that a virgin DM should know?"
If everyone has fun, its a good session.
Don't get locked into a plan.
Just because you figure you are going to have to drop it half way through the session, is no excuse for not having a plan. Grab the plan when things are moving slowly. Drop it when the events start to have a life of thier own.
Roll up random encounters ahead of time, or don't use them at all. Don't let the dice run your game.
Extemporeous is best when it is an illusion brought about by your preperation.
2) How exactly do you DM a good session?
Prepare. Prepare. Prepare.
Relax.
Laugh. Smile. Joke. Tease. Entertain. Keep your players focused on what they should be doing. Don't let them get so distracted that the game starts to fall apart. Encourage them to be IC all the time. Encourage them to plan ICly. Encourage them to address each other ICly. Encourage them to yell there war cry and charge into battle ICly.
"3) Would you recommend using a module at first, just to get used to DMing?"
A module isn't a bad idea, especially if it is a good one, especially because a good 1st level module is the hardest type to write.
However, no module can be ran well right out of the wrapping. Any designer will tell you that 64 pages (or less) is terribly constraining and that alot of how he ran his own module couldn't make it into the text. You will need to flesh the story out usually, and in many cases add depth to the introduction and/or the NPC's. You will also need to go through the module and determine if the ammount of magic/treasure being given away corressponds to your desires for the campaign. I've seen supposed first level modules giving away +4 weapons and the like in earlier additions. I tend to read through modules and annotate any room that I think needs amendment, or make notes to clarify ambiguities or missing details. Go ahead and roll up random encounters ahead of time, and detail them sufficiently to run them seamlessly.
"4) If the players are more experienced than myself, will that matter?"
Unfortunately, yes. They are going to be very frustrated if you don't know what you are doing. Make sure you have the basic rules more or less memorized and keep tables handy until you reach the point you are sure you don't need them. Nothing is worse than a referee that doesn't know the rules of the game he's running. You have to earn thier respect.
But just because you are new, don't forget that you are in charge.
Lastly, it is a sad truth that many experienced RPers are worse players than many neophytes. Experienced players tend to spend too much time meta-gaming.
"5) What's the hardest/most time-consuming/disliked aspect of DMing?"
What isn't time consuming about DMing? Most of the best DM's I've ever met struggled with real life because DMing consumed too much of thier time. Remember, it's just a game. Keep your life in perspective.
Try not to flesh out the whole world before you start gaming. Start with one little corner and some ideas and start painting outward as needed.
I love to DM, so there is little about it that just repels me. Writing up original room descriptions is probably the most time consuming part. Achieving a deep story while keeping the players full and active participants is probably the hardest part.