Basically Mass dispel is an AoE cleanse. Normal clease is generally on a 60 seconds cooldown. 10 seconds extra to make it AoE seems fair, even though it's an ultimate. Being able to team cleanse Horrify (80s cd) and Mosh pit (120s cd) since Anduin himself could probably avoid them also seems pretty strong. It would also completely nullify mainly Leorics tomb (50s CD), and Sonyas leap crater (70s CD), and to a lesser extent Tassadars walls. making those talents not completely useless...