could be because the original photo is not found and what you are seeing is the enlargement of the thumbnail picture.
The exclamation point indicates that iPhoto has no idea where the original is.
iPhoto 11 is the current version of iPhoto, it may be worth springing for that but, in the meantime, drag out your trusty Time Machine backup and go back to when all the pictures were good.
I have iPhoto "09 and over 2,000 photos. My iMac has OS 10.8.2. Going back to my iPhoto library, I discover MANY photos--that were sharp before--are now fuzzy and many others show nothing but an ! (exclamation point!!). No people or other scene. The photos are shown fine in the small photos but when I click enlarge that's when the fuzzy or ! appear.
Odd that photos next to each other can have one fuzzy and the next clear.