Every form of non-champion damage that contributes to high burst in the game:
Items:
items,
,
,
,
,
,
,
Not counting Duskblade because a huge chunk of its damage has been converted into utility, granted said utility greatly encourages bursting.
Not counting IE/Deathcap because they're often irrelevant most games to most champions until the game gets to the 35+ minute mark.
Keystones: Conqueror (with minimal effort from certain champions), PTA, Lethal Tempo, Hail of Blades, Electrocute (granted less of a problem now), Dark Harvest (again - nerfed recently), Predator
Runes: Last Stand, Coup De Grace, Cut Down, Cheap Shot, Sudden Impact, Zombie Ward, Ghost Poro, Eyeball Collection, Ultimate Hunter (with ultis like Karthus's), Absolute Focus, Gathering Storm (granted typically irrelevant in this short game meta), Shield Bash
Summoners:
,
(the second one on a team),
(hey, at least it's nerfed on PBE?)
Monster buffs: Infernal (stacks up to three times), Herald, Baron, Red Buff
IMHO, either the majority of AD/AP burst items need to be toned down/reworked or the damage from Runes/Keystones needs to be toned down/reworked. This all just seems like too much considering many champions' burst damage is already pretty high.
I mean, just look at all of this - since when was there THIS MUCH bonus damage and %damage amp in this game?