Complex grouping is basically Estidn's idea, and that prevents powerlevelling, but here are the cons.
-Groups have to go level from mobs around the highest player's level, that means there is an increased chance in getting hit and dying if there is a wide level difference, which leads to
-Players grouping around the same level.
-Boss mobs like Voltron won't be worth killing, if say, 75M xp gives only 12.5% of the highest player's TNL, and a lowbie priest gets only 25% of his TNL, which could be a mere 10-20M.
But..instead of the highest level player getting half of the total xp he would get if levelling himself, the formulae would be 0.50X + ( (number of PCs in a group -1) * 0.1)X, where X is the XP he would get if levelling by himself.
However, this formulae would only work if the mob's level <100. If the mob is > level 100, then the formulae for normal grouping applies.
*On second thought, I'd go for normal grouping...unless there is a formulae that accounts for experienced players showing newbies around zones with 'follow'. If you're passively in a group, and leading a newbie to kill low level mobs, and that newbie does all the damage, he should get all the XP.