From the League of Legends Screenshots topic:
Rengar's Q crit was only partially removed with a recent bug fix - since it's a next-attack modifier (changing the damage of the autoattack itself) as opposed to on hit effect like Decisive Strike, it works a bit differently in terms of crits. It had a base damage and an Attack Damage portion; the base damage no longer crits, but the Attack Damage portion still can. This is how the crit works with Savagery as I understand: Base damage: 20 x Savagery rank (Empowered: 15 + [15 x Rengar's level]) This does not critically strike Attack Damage Ratio: 100 / 105 / 110 / 115 / 120% (empowered: 150%) This can critically strike I did some math at work today, and with an infinity Edge, an Empowered Savagery that crits deals 15 + (15 x Rengar's level) (+375% AD). Let's take that with a level 18 Rengar with 300 Attack Damage. 15 x 18 = 270 300 x 3.75 = 1125 15 + 270 + 1125 = 1410 physical damage So an empowered Savagery that crits deals 1410 damage. If it doesn't crit, that number falls to 720.
If the rengar buys a Trinity Force, Spellblade adds 218 damage (Rengar's base AD at lv. 18 is 109). That damage doesn't crit, since it's an on-hit effect. In comparison, a normal Savagery that crits with the same stats deals 1000 damage. If it doesn't crit, it does 460. The Empowered Savagery crits for massive damage, and since most rengar players get more than that (between IE / BorK / LW / Triforce it ends up around 300) that damage just keeps growing. 400 AD? Empowered Savagery crits for 1785 total damage. Now you may ask "how is that balanced?" While those numbers are massive, you also gotta remember that those numbers don't factor in armor/MR, and most squishies will buy armor / a Guardian Angel against a rengar that does that kind of damage. A rengar that builds like that would spend half his ult's time praying to the RNG gods as most assassin rengar builds have between 40-50% crit chance, tops. Also if he invests that much in damage, he'll be squishy and downed in no time in teamfights against decent players. SIDE NOTE: IIRC, only Blitzcrank, Jayce and Jinx can fully critically strike with their enhanced Autoattacks (including bonus damage), since their modifiers multiply the damage dealt by the attack rather than working with base damages and ratios. Rengar used to be able to, as did Nasus and Garen, but not anymore.