First off I would make sure to go with one of the more popular distributions, and one that is active. Going for a small or inactive distro can be frustrating as it may have more bugs and other issues along with less support. Though a solution for one distro is usually applicable to other distros. The page hits ranking on the right hand side of
DistroWatch can help you find out which distros are popular.
Next I would discount distros like Gentoo and Arch unless you really like using the command line and want to setup everything yourself manually. Even though they have some really good wiki's for setting pretty much anything up. Though whatever distro you choose, you should be prepared to have to open up the terminal sometimes.
Also I'm assuming that you don't have some very specific task that requires a certain distro, and that you just want to try out Linux to see what it's like.
To add just that little bit more confusion you may see some distros described as being Gnome or KDE based. These are two of the main desktop environments for Linux. For example Ubuntu is Gnome based, whereas Mandriva is KDE based. In general I tend to think of Gnome as looking and feeling a bit older but being more stable, whereas KDE (version 4) is chock full of transparencies and tends to be a little less stable. Being Linux there are of course lots of other options... I would start with Gnome.
In this case distros like Ubuntu (or Mint), Fedora, openSUSE and Mandriva can be good choices, at-least for an Intel computer. If I was starting out with Linux now I would try Ubuntu first, or maybe Mint which is Ubuntu + a load of extras. Though I like Fedora myself, even though it can be a little more hassle to setup, also Fedora has a current PowerPC release though I don't know how good it is.
If you have VMWare Fusion or Parallels then it might be nice to try running it using them. If you don't have ether of them then there is always
VirtualBox which is free. This way you would be able to update everything and try out installing some other programs, without worrying about messing up OS X. Though you won't get to see any of the 3D desktop effects.
As for the misconceptions. Choosing your first Linux distribution is often quite difficult, and there are a lot of distros which are very similar and are aimed at the same task/people. So you weren't completely wrong.