This is Gonna Hurt - 10000 XP
Complete any Campaign Mission on Heroic or higher difficulty with the Famine, Tilt and Thunderstorm skulls.

Shutdown Slamdown - 30000 XP
Complete Shutdown on Legendary difficulty with the Mythic, Tilt, Tough Luck and Thunderstorm skulls.

Spartan Ops

I am the 99 Percent - 20000 XP
Kill 40 Elites.

Destroyer of Grunts - 10000 XP
Kill 70 Grunts.

War Games

Spartan Takedown - 20000 XP
Kill 150 Players.

One, Two, Three, Four... - 10000 XP
Earn 30 Multikill medals.

Click here to see a complete list of this week's Weekly Challenges.
What's the deal with the massive xp bonus for all challenges? It doesn't do much for those of us at 130 unless you guys are keeping track for specs that will be released down the road. Or is it simply a reward for those who have stuck around?