To change the default font in Illustrator from Myriad to something else you prefer is fairly easy, although not nearly as straightforward as it should be. All you need to do is locate the following folder buried inside your home directory:
Users > *username* > Library > Application Support > Adobe > Adobe Illustrator CSX > en_US >
These are the base template files that are loaded each time you start a new Illustrator document. You'll need to change the ones you use most often. If you're like me and *luckily* have no clue what things like *Flash Catalyst* even are, then you can leave those alone.
Most of the time I just start with the Print template and occasionally the Video and Film one.
Open up Basic RGB.ai (or whatever template you want/use)
Go to Window > Type > Character Styles
You'll get this window, double click on *Normal Character Style*
Now under *Basic Character Formats* you can set your new preferred default typeface.
Save and Close this template.
Now, try launching a new document using the Basic RGB template (or whatever one you've just changed) and the default font should reflect your changes.
Another thing I found this useful for was tweaking the Video and Film template. I usually avoid using it because the transparency grid and those rulers/guides it has on by default I would just turn off. You can tweak these templates to your hearts content, so for starters I just turned off the transparency grid and made some changes to the guides to better suit my taste.