The show after Season 3

Excluding the obvious departure of Prue and the addition of Paige (both characters who I thought were great), what happened to the show's environment after Season 3? I know that there was some serious shift on the show's texture after the third season, but I can't really explain clearly how the show seemed to changed greatly in Seasons 4-8. During Seasons 1-3, the show and its mythos felt very intriguing, but somewhere after the third season (maybe moreso after fourth), the show started to become...less intriguing, I guess. I don't really know how to explain it, but maybe someone else can explain it better than me.