can someone explain to me the new party exp thing?

The old experience formula distributed exp by two factors: How much damage you did and what level you were. So does the new one.
Old one multiplied the ratio of damage and level by .4 and .6 (not sure which is wjich).
New one multiplies them by .2 and .8, making damage done a larger factor in the exp splitting.

inb4Russt & his corrections

Actually it's the old split that was 20% damage, 80% level relation. Now it's 40/60. Even the damage part was still relatively new, it used to be that the 20% was based entirely upon the person who did the most damage/received the last hit.