Here's another story and another fix: I have two, mid2011, MacBook Airs (an 11" and a 13"). With the change to Mtn. Lion, my shutdown times went from 2-3 seconds to 12-15 seconds.
When I examined my system log during the shutdown process, I noticed that a couple of different background apps related to printing to a Canon Advanced ImageRunner copier were trying to shut down. I had completely forgotten that I had installed printing and faxing software a year ago in an experiment. This high-end machine is used by my workgroup at the college I work for and and I also have access to three different laser printers. As it turned out, I didn't ever print to this copy machine. Also, this software is of the the type where I don't get automatic update notifications. And, the software was out of date.
To make a long story short, I used FindAnyFile (my favorite search software) to locate all the Canon software on my two computers (I don't use any other Canon products) and then I deleted it. Problem solved. Since doing this, my shutdown times are now back in the 2-3 second range.
What was particularly odd about this was that my system log was showing the shutdown process was finishing in about one second when the actual time for the screen to go black was 12-15 seconds. In hindsight it seems apparent that the system was trying to shutdown the two Canon background apps but that they weren't quitting like they should.
While I doubt that many users have Canon ImageRunner software installed, perhaps examine your system logs for apps (particularly backgroup apps) that are being quit in the shutdown sequence. Then, do some investigating as to the source of these backgroud apps and whether the software sources are Mtn. Lion compatible.
You can examine your system log by using the Console app in your Utilities folder (which is in your Applications folder). To focus on the shutdown process, change your system clock to show seconds. Do a shutdown while noting the exact time (with seconds) when you do the actual shutdown. (There's another more sophisticated way to accomplish this, but this works if you aren't familiar with Console.) When you shutdown, have Console open and click the box to reopen windows when you reboot. Wait a minute so that the shutdown time is significantly different from the bootup time. Then, reboot. Console will open again and begin examining the system log at the time you shutdown.