They wait until new part of gameplay from Niantic :
- missions (not only reserved to the tutorial part)
- achievements and badges (à-la-stackexchange ;) )
- new items
- new levels (9 to 16)
Apart this teasing, for the moment when reaching the level 8, you gain the ability to use the mass destruction weapon that are L8 XMP Bursters (2700 damage around 168 meters) ! No portal can resist to this (a full shielded L8 portal can be destroyed with 4 XMP L8). But the levelling up driving force is dead, you'll have to find new motivations.
To have such powerful items as L8 XMP, you'll have to build powerful portals (at least L6/L7), and to build such powerful portals, you'll need many powerful teammates: you need to be 4 L6 to make a L6 portal, you need to be 6 L7/L8 to build a L7 portal and 8 L8 to build a L8 portal.
So, one good way to use this power is to help your teammates to level up. For example, a L8 player can destroy portals (or take them down to 1% power), while lower levels finish the job and rebuild the portals to gain AP.