Shortly after starting college I heard about the wonderful strategy / ideology of open source (hey, I'm from the sticks, OK?). As I recall, I'd never seen a machine running GNU/Linux before. Being an over-the-top Windows tweaker (Windows 98 broke completely about every two weeks), all that control sounded like gravy to a fat f***.
So in 1999 I installed Linux for the first time; SuSE 6.3 as I recall. The PC was brand spankin' new, and I had some trouble setting up the network card. After a few days of emails*, dependency hell, man pages and make menuconfig, I went back to Windows. A friendly voice in the back of my head told me to "Please try again later."
My next attempt was with Mandrake, which was said to be a lot more user friendly. I was told it would install all my hardware automatically. Sweet! Turns out that was the first time I heard this lie which some users perpetuate even today. I didn't have the stomach for another dead end, so it was gone after half a day of fiddling.
At this point I'm a happy Linux user (desktops, server and laptop), only switching to Windows for games. It's a lot more productive than Windows, but remembering my own problems I'm careful when recommending it to others. Don't lie to newbies. They will not forget it.
Corollary 1: Don't tell users that all hardware works on Linux (there are lists, however), and then blame the vendor for not supplying an open spec. Users care about working hardware, not excuses (Hi, Apple! Wanna downgrade my iPod for me please?)
Corollary 2: Linux software has lots of bugs. Sure, high profile software like Apache, Emacs and Firefox are close to infallible (not counting third-party extensions), but there are still crashes, wildly incoherent interfaces (look at media players, for instance), outdated or missing documentation documentation (yay FreeBSD), interoperability issues (one format per program and few translators) and more.
* At this point it would be extremely rude not to give eternal thanks to all those who have been willing to aid a stupid (and often angry) newbie to grasp Linux enough to finally switch to it for good a couple years ago. Thank you all!