Keys
Rare (gold) drops
Special drops (Shamrock Ale and Four-Leaf Clover)
Rare Chest Loot
Access to The Fissure of Woe and The Underworld
I would have included green drop weekends over chests, but I got very frustrated with the last couple of those. Chest drops make for an expensive weekend event, but I generally benefit from them.
Here's a couple of suggestions:
Increased Rare Material drops and salvage chances. I'm currently thinking about getting Vabbi armour for a couple of my characters, so that would be very useful.
Also, I agree with the comment above - use gold/green drop increases to encourage teamplay. Have drop increases that only kick in when a team with at least four human players are playing a mission. Maybe it could be a weekly thing, with each campaign getting a handful of highlighted "focus" missions (say 3, spread throughout the chapter) and each weekend the focus moves to the next set of missions in sequence. That would get everybody playing the same set of missions each weekend and probably help anyone who's stuck in some otherwise deserted or "herocentric"

part of GW and struggling to find a PUG.