Reading all of the posts about Oktoberfest brings back some great memories. Here are my thoughts. I've been 4 or 5 times with my brother (in our mid to late 30s) and a couple times with my Mom when she was 70 (raised in Bavaria).
It's all about expectations so its good so many are doing research beforehand. I added some FAQs below to help with your plans, or just to get you more and more excited for your trip.
How can I get a seat in one of the tents? Show up early; around 10am. We've tried to reserve a table a couple times 10+ months out and never received a response. We would get up, grab some food from the local Aldi and head to the tents. Sometimes they check your bag when you go into the tents so you may not to sneak some snacks in. We've always had success though. Okay, back to the seating situation. Others have said you should be able to find a seat arriving by 1pm during the week on a non-holiday. That has not been my experience. You may find yourself wandering around the tents for hours asking to join their table. It can be a huge pain. I recommend arriving around 10am to not deal with this, especially if its your first time to truly enjoy the Wiesn experience. I you arrive late or can't get into the tents, it's easier to find a seat in the outside area of the tents (most have areas outside). You hear the music through speakers but its not the as,e as being in the tent. You can still have a lot of fun as its still a really good time socializing and meeting others at your table.
Which tent should I go to? Are some more fun than the others? They're all fun. You will read some are party tents and some are younger crowds, etc. My experience has been they're all similar, especially the larger tents. I honestly couldn't tell the difference between some tents. The type of music played is different between some tents. Some have more traditional music and others more modern music. Just like any band, they take breaks and when the come back to play, the crowds really enjoy it. The later in the day, the more everyone gets into the music; of course as the bier flows. Everyone enjoy the music and totally gets into it after a few liters. As others have said, they all turn into big parties after the dinner time (around 6 or 7pm). You'll see everyone standing on the benches singing along and just enjoying everyone's company. Standing on the tables is frowned upon so don't do it. Even earlier in the day, I've seen security tell people to not stand on the benches - they stop enforcing that when you have 5K people doing it. I just wanted to try some different tents so I went to a different one each day. We even lucked out a couple times and switched tents and still found a seat mid day. It's funny since you'll have some locals tell you "their" tent is the best or this bier doesn't give you a hangover like the others. One said they always go to Hacker because their bier is the best and doesn't give you a hangover like Spaten. After a couple liters, many don't care about which bier they're drinking. And for hangovers, it's about quantity, not quality :) After 4 liters, even if its spread across all day, you'll feel at least a little worn down the next day.
How does the beer/food/service work at the tables? Unless you have reservations with vouchers, you just pay as you get your food/bier. Bring cash- there are ATMs in some tents and outside but they can be a hassle so just plan ahead. If I didn't have cash before I would arrive, I always hit the ATMs near the main entrance. There may be a line but not a problem. Bier is still under 10€ so just give them 10 per bier and they keep the change, especially now I think they are between 9 and 10 for 2013. Prices are a little different between tents but not a huge difference. Even when it was in the 8€ range a couple years ago, we would still not expect change back. Maybe it is was exactly 8, I would give a 1€ tip. Anyway, no need to worry about that any longer. It will be interesting once it's above 10€ since you'll be getting a lot of change with each liter unless you carry around some 1s and 2s. Until the evening, the servers would come around and take your orders for biers. Later on in the evenings, they'll just bring a bunch to the tables and you ask for one.
My experience is you must have a seat and be sitting to get a bier. It's not like in other bars where you can be standing around and get served or go to the "bar" and order a bier. Later in the evenings when everyone is socializing and standing up, it's much easier to get served if you're not sitting.
What should I wear? It's very casual and it really doesn't matter what you wear. You'll of course see many in dirndls and lederhosen but also a ton in just regular shirts and jeans, pants, or shorts. Dress comfortably and just have a good time. It's fun to see what some wear. You'll see some in costumes like Toy Story characters a few years ago and other popular cartoons, etc. some Canadians will wear hockey jerseys too. Last year we met this group of physicist Americans working in Switzerland who planned to wear flight suits the next day. They aren't fighter pilots but just thought it would be cool to get the attention from the girls. They actually reminded me of the guys on Big Bang Theory. Anyway, we planned on meeting up with them the next day when they were Im their flight suits. We saw some of their friends, the ones without flight suits. Security wouldn't let them in. Haha. It was funny and a little sad since they all had it planned and were so excited. We saw some other in flight suits so just depends I guess. Security may have thought it looked too much like military uniforms and probably didn't want the hassle of potential issues.
If you want to get traditional dress, you can go to a local store. We bought some traditional button down checkered shirts at Angermeier (google it if you want more info).
What is there to do in the tents? Drink bier, eat some food, and socialize with others at your table. It can be odd to think that's what you will do for 10 hours if you arrive at noon. Of course there's the music and dancing and singing along too. It's really a fun time.
What about outside of the tents? It's a big carnival so a lot of rides. Some are pretty cool and crazy that they are temporary. I think it's about 5€ per ride, give or take a little if I remember correctly.
Where should I stay? You've read a lot about this by now if you've checked out most of this thread. I would hate to pay 300€+ per night so we always use hotel points at one of the Sheratons. Like many have said, you can stay outside of Munich and just take the train. It's very convenient.
Is it safe? Yes, it's very safe but just like any travel/festival experience, be aware of your surroundings. Keep money is a safe place (not in your back pocket). Everyone is really friendly but of course with so much bier, you're bound to see some altercations. It's almost guaranteed to see a fight in the tent if you're there past 8pm. Security is very present in the tents and Polizei outside the tents. If things get hairy enough, the Polizei come into the tents. If you see a couple guys getting heated and think it will escalate, just walk over to a security guard and let them know, they really appreciate that. Some of the fights we saw were the worst fights I've ever seen. Broken krugs and blood all over. Of course they will all get kicked out and then that opens up usually a whole table to sit at, hahah. Like I mentioned, fights happen but I still feel very safe. Just be aware of what's going on around you. Don't try to break something up, just get a security guard and let them do their job.
Other random thoughts: it's crazy how much bier some kids and adults drink. I say kids when someone is usually under 20. You'll see many bring their own and sit on the hill right next to the Wiesn and drink beforehand. Then you're guaranteed to see someone passed out by early afternoon. Many retire to the hill mid afternoon and you'll see many "resting" there. There are great pic opportunities there. :) Some walk down the hill and get more and more speed since they don't judge the angle correctly. You can guess what happens next. I've seen some guys(and girls) totally faceplant and get hurt. If you've never been you have no idea about the hill. If you've been, you can totally envision what I'm talking about. You'll also see some totally out cold passed out in the middle of the tents at a table. Their friends usually give them a hard time about it.
Okay, I rambled enough. It's a fun time. I'm going there this year again with my other brother and a friend who both haven't been and really looking forward to it. Enjoy, all!!