Here's a good article describing the added features of CM7 over vanilla Android.
CM7 Settings for Phones - Explaination - Blogs For Android
Many of these features may be of little value to you, but I like to point out a few that should be considered by everyone:
Revoking individual permission -- With Android, permissions are a all or nothing situation, you see the permissions an app requires, and you either accept them all, or you don't use the app. With CM7, you can disable individual permissions on any app, rather than having to accept them all. For example, you download a game app that wants to access the internet for whatever reason (stats, hi-scores, tracking, etc). You don't want to let the app access the internet, so you revoke that permission, and keep on using the app.
Status bar & Notification -- You already know how your status bar and notification dropdown looks and acts on plain Android, here are a few of the tweaks on CM7:
- Swipe the status bar left or right to change your phone's brightness
- Adds a widget with buttons that toggle various system settings (wifi, sounds/vibrations, etc)
- Allows you to swipe away individual notifications, instead of having to clear them all
- And on my BACKside builds, you can center the clock and change it's color
Lockscreen options galore -- You can change your lockscreen to many different styles, with different unlocking options, and automatic app launchers. You can even remove the unlocking control buttons and use gestures to unlock the phone and automatically launch apps. For example, you could draw a "u" to unlock the phone only, an "m" to unlock and go to email, an "s" to unlock and go to sms messaging, etc etc.
Performance and battery -- With CM7 you can overclock/underclock your phone without extra apps to make it run smoother or save battery life. You can also change the way the screen renders to gain additional performance or battery life. And you can change the way memory is handled.
The article I linked to above has four pages of CM7's extra settings. To get similar extra settings on stock Android would require so many apps you wouldn't have room left for any other apps.
The only true way to understand all the hoopla about this is to try it out. Bottom line is, if you do flash a CM7 ROM and follow the install instructions, you will create a nandroid backup in recovery of your current setup. If you don't like CM7, you can restore that nandroid backup in about 5 minutes, and your phone will be right back to the way it is now!
Deja Vu. 
