Oktoberfest in Germany - what to wear

Traditional lederhosen is acceptable:
http://www.cnn.com/2013/09/20/travel/oktoberfest-2013-9-rules/

Gird your bosom, hitch those hosen

Worried that squeezing into a bosom-lifting dirndl or a pair of
skin-tight lederhosen will make you look ridiculous?

Don't worry: it will, but considering almost everyone will also
resemble an extra in a B-grade medieval romp, you'll fit right in.

To put it another way, when in Bavaria, do as the Bavarians do -- and
they're pretty proud of their huntsman-and-strapping-maid heritage.

Rent a costume if you don't fancy splashing out on your own outfit.

http://www.yr.no/place/Germany/Bavaria/Munich/statistics.html
September Normal: 13.4°C High avg: 19.4°C Low avg: 8.8°C Days precipitation with in month: 9

Or order it direct from the Oktoberfest on-line shop.
http://shop.oktoberfest.de/en/Tracht-for-Women/
The big department stores by the Hauptbahnhof will have plenty of stuff too.

What you would wear to the state fair or fall fair/festival because it's essentially a nicer version of a midway. A lot of locals come after work and are dressed in traditional costume or just the same casual clothes your average American wears on a fall day. It seems to be tradition that many workers you'll run across are wearing their traditional attire even if they are not on the Oktoberfest grounds. Comfortable shoes, but I wouldn't overthink it. :) Prosit!

If you want absolute, top-of-the-line Tracht (traditional-ish clothing) and money is no object, visit Grasegger in Garmisch-Partenkirchen. You can get cheaper stuff in Munich, however. There's a large store on the pedestrian shopping street between the Hauptbahnhof and Marienplatz.

Wearing Tracht, of course, is not required. Wear whatever you find comfortable.

or just the same casual clothes your average American wears on a fall day. I would say, not quite. Except in really hot weather, Germans usually don't dress as casually as North Americans when they leave the house. Meaning, don't expect to see anyone in ratty jeans, sweat pants or old Tshirts.

Weather is still usually warm and on average, it's the driest time of year. This doesn't preclude the possibility of rain on any given day, however, but given the roll of a dice on a long-term weather forecast, you are more likely to stay dry.

Tom, I live in the sheltered world of LL Bean and Lands End land--to me that means average American.. But, yeah, I get that a lot of people dress sloppier than that.

Last time at Oktoberfest, I greeted a group of three couples "I can tell you are Americans by the way you dress." Jeans, casual shirts, very LL Bean. They all laughed.
"We are from Austria!"
"We are from Switzerland!"
"And we are from Munich!"
So much my fashion sense.

You would NOT be out of place in traditional Trachen. You will probably need a light jacket in any case.

Just check the weather forecast in early September and adjust your packing as needed. Whatever walking shoes you would wear at home for that weather -- if you're concerned about rain, you can try a waterproofing treatment for your leather shoes.