The main thread on SMF where the various themes are posted is at:
http://www.simplemachines.org/community/index.php?topic=15620.0The reasons I picked SMF were the following
1. The theme I found was the nicest of all of them for what I wanted (something unique, simple, and quick loading/light on graphics)
2. The administration area was easy to understand, easy to make changes, and full-featured at the same time.
3. The forum appears to not be intensive for load issues.
4. The forum has an active development team who are dedicated to improving it and keeping it secure.
5. Customizing it has been easy. I was able to determine where things are located to change colors, add a simple graphic, etc. without a lot of hassle.
Now after doing an upgrade today, I would also add:
6. Upgrading is very easy. I lost no changes I'd added, and the upgrade itself took less than 5 minutes (the site backup took far longer to do because I wanted that in case anything went wrong).
SMF seems to have a lot of great features in the admin area similar to what a paid forum like IPB or vB has but without the price tag. Unlike those 2, it also appears to be less intensive and faster loading. Of course, the theme I picked is really a minimalist one, so it is going to have vast time improvements over some other themes.