Infinity Edge is slightly weaker now (little less AD), we shifted power elsewhere so it's less automatic and so that we could add cooler things to the Marksman space.
Essence Reaver scales its CDR off of your critical strike chance so that it works as a big core item (think IE space). It can now give upwards of +30% CDR.
Phantom Dancer does stuff other than just be straight up damage. Bonus movement speed while near your opponents and you take last damage from the last champion you hit. Isolate and kill people, basically.
Rapid Firecannon charges up like Shiv, but when charged instead of prepping you for the thunder, increases your attack range and gives you a fiery hit at the end.
Shiv does even more damage versus minions. It's specialized further into being the bursty clearing tool.
Hurricane now crits and the bolts crit. Also movement speed. It's in the PD / Shiv space now, so it should be an option for more Marksman.
Executioner's Calling is back, but it's a bit different and cheaper. Also it's a component item. It builds into something new.
Last Whisper is now a component item. It builds into new things.
There's something called Giant Slayer. It hurts health dudes.
Merc scimitar has Lifesteal, you should feel comfortable taking it over BT.
Death's Dance exist. It has something like Lifesteal but better. Also it "delays" damage.
As an adc main, i'm loving this. I can finally play the Rat. Twitch with the new pd hurricane and giant slayer item. Them teamfights, them assassinations.