* Dalingrin's overclocked kernel is very nice- it can overclock from 800mhz to 1.2ghz stably, and 1.3ghz slightly less stably. Works on all 2.2/2.3 ROMs (Stock 1.2, Nookie Froyo, CM7), and is included with recent CM7 nightlies: http://forum.xda-developers.com/showthread.php?t=925451
* The app Nook Color Tweaks (http://forum.xda-developers.com/showthread.php?t=1104039) is also very nice- if you have Dalingrin's kernel, it'll let you customize the voltage settings for overclocking, increase speaker volume, and this:
* USB Host support. If you buy a microUSB male to USB female adapter, you can plug in a mouse, keyboard, game controller, webcam, flash or hard drive, what have you. Works extremely well. http://forum.xda-developers.com/showthread.php?t=1105497 (note that the Nook Tweaks app with the Dalingrin kernel already has this built-in)
* CM7 (and maybe Nookie, not sure) have Bluetooth support, meaning that you can use a BT headset or controller (Wiimote, mostly) with it. The main downside is that it's kinda wonky- it works most, but not all of the time, and it has a really terrible range (I think 11 inches was the last I heard). However, when it works, it works fabulously.
* In CM7, if you'd prefer using the internal 5Gb of storage as opposed to a microSD card, flash this in recovery: http://forum.xda-developers.com/showthread.php?t=1034939
* The Netflix app won't show up in the Market by default, and even if it does, browsing the menus won't work in landscape. Grab a fixed APK from here: http://forum.xda-developers.com/showthread.php?t=1159266
* If you want to get Hulu working, look here: http://forum.xda-developers.com/showthread.php?t=1040971
* There are a few apps that crash if you don't have an IMEI, which the Nook Color does not. It's not a fantastic idea, but here's an IMEI generator: http://forum.xda-developers.com/showthread.php?t=1103766
I've had a CM7'd Nook Color for a few months, and it works fantastically. As long as you're not gaming and don't mind the lack of webcam, it's better than an iPad and half as expensive.