1. Expect a few bugs
When a new upgrade is available, expect a few bugs and flaws. These bugs will be ironed out after the initial release. Apple releases updates regularly to fix bugs so it is worth waiting for stable software while Apple fixes its flaws.
2. Downgrading may not be possible
As with all iOS updates, downgrading is not possible. Considering the hype surrounded by the iOS 7, may be this upgrade is here to stay
3. Some apps may not be compatible immediately
Developers had access to iOS 7 much earlier so that they could work on updates for their apps. However, as much as iOS 7 is ready for launch, not all apps have their updates ready for launch. Hence, all your apps would not work the way they used to work before.
4. You will lose your jailbreak
If your device is jailbroken on iOS 6, then the jailbreak for iOS 7 may not be available instantly.
5. Your older device may not be compatible
At the moment, developer reviews are available but it would be wise to wait for user reviews before updating.
Delete any unused apps and make space for the new update