Grabbed this from a post on summoner school:
Core damage items
Infinity Edge: slightly cheaper, loses 15 ad. Still good on champs who don't rely on their abilities for dps.
Essence Reaver: same price as IE, gives 20% crit and 10% cdr on its own. It also gives cdr equal to your crit chance from other sources up to 20% (so a total of 30% cdr). Essentially, this trades the 50% crit damage from IE for 30% cdr. Most champs will hit 50% total crit, so ie's crit damage boosts your total autoattack damage by ~17%. On champs who rely on their abilities for dps (tristana, sivir, ezreal, etc), the cdr from reaver could easily outweigh crit damage from IE. That said, I wouldn't make a firm statement here until after a lot of playtesting.
Overall, infinity edge was split into "ability focused IE" and "autoattack focused IE".
Phantom dancer: 40% attack speed, 30% crit, reduce damage taken from the last enemy you hit. Essentially, the duelist's item. Probably good on vayne/other splitpushers.
Statikk shiv: 35% attack speed, 30% crit, different lightning damage. I have no clue if the proc mechanics changed. It no longer builds out of avarice (I don't think avarice exists anymore). The waveclear item. Build this if you want waveclear (shocking, I know).
Rapid Firecannon: 30% attack speed, 30% crit chance. It stacks up like shiv, but you get 150 bonus range and some single target on hit damage when it procs. I don't know details of how the proc works. This is the sieging item. Stick it on someone like sivir and she can suddenly hit turrets from a reasonable distance. Cait might outrange turrets (800 total range). It might also be nice for short ranged carries in teamfights, but the uptime probably isn't that great.
Hurricane: This is now a zeal item. 35% attack speed, 30% crit. The extra bolts deal less damage but can crit. This is the teamfight item. Extra crits, extra on hits, and you don't trade away single target dps. This will probably be amazing on jinx/kog.
All items except pd cost 2500g (pd costs 2700g). Overall, you can choose between them based on what you want to do. Need more waveclear? Shiv looks nice. Want aoe for teamfights? Hurricane is calling your name. Some champs will have particular synergy with one over another, but all will be pretty viable depending on the game/team comp.
Bloodthirster: 200g more expensive, lifesteal isn't unique, ad reduced to 75. Still the same basic item. It will probably be best when paired with IE.
Blade of the Ruined King: 200g more expensive, unique passive nerfed from 8% current health to 6% current health. This is now a dedicated counter to health stackers. Before, it was reasonably efficient even against squishies. Now, you probably want to pass unless the enemy team is really tanky or you have insane synergy with on hits (cough cough kogmaw).
Death's Dance: 3400g (same as bork), 10% cdr, 12% "lifesteal" that applies to all physical damage, and 20% of damage taken is instead taken as a bleed effect over 5 seconds. The ability focused version of BT. If you grabbed reaver earlier, you want this to hit 40% cdr (and "lifesteal" that applies to abilities is useful on most of the champs who would want it).
Mercurial Scimitar: 75 ad, now has 10% lifesteal. I have no clue how this item will turn out. In theory, this can replace both a defensive item and a lifesteal item now. Will people get "double" lifeteal? Will they grab a second zeal item for all of the crit chance? Will they just tank up with a banshees? Have fun playtesting it when you get a chance.
Armor pen items
Last whisper is now a component. It builds into at least two different items.
Lord Dominick's Regards: 40 ad, 40% bonus armor pen (unique), up to 15% bonus damage against targets with more health than you. Dedicated tank buster, pretty straight forward. Use against health stackers.
Mortal Reminder: 40 ad, 40% bonus armor pen (unique), greivous wounds. The mundo/voli/etc counter that you have been waiting for. If the enemy team has a lot of sustain, this is the item you want.
Youmuu's ghostblade: 65 ad, no crit. The price was increased to 3200g. This is no longer a replacement for a zeal item. However, the two last whisper items give bonus armor pen, not total armor pen -- they don't do anything if the target isn't building armor. Against low-armor teams, youmuu's might be efficient as a replacement. This will need some testing.
Honestly, people might just skip this entire category when against a squishy team. Ghostblade will probably be reasonably efficient, but double zeal items or the like might be better.
Random other items
Trinity Force: less attack speed, less movespeed, 5 less ad, no ap. However, it has 20% crit and 10% cdr. It is the same basic item, and it will be used on the same champs and built at the same time (ie usually rushed). The changes make it synergize really well with essence reaver (you hit 40% cdr at 2 items), and the extra crit is nice with IE as well. Trinity/ER will be very viable on ez, and trinity/ie will probably be effective on corki (I don't think he likes cdr enough to make trinity/er better, but I could be wrong). While it is technically a zeal item, it doesn't really fit with the other zeal items (different stats, different build order, etc).
Guinsoo's rageblade: gives stacking ap/ad/attack speed (like sanguine blade). At full stacks, it also gives some on hit damage. Not really an adc item, but it might be viable on kog/corki/ezreal?
Maw of Malmortius: loses the bonus ad when low on health, but you get attack speed/lifesteal/spellvamp when the shield procs. Might be a viable defensive item? Who knows, people might run maw/mercurial against heavy ap comps and semi-sorta get double defensive items and double lifesteal (in a weird sort of way).
Overall, item builds will be much more situational. There are a lot of item builds with very similar base single target dps. The key is choosing the items that enhance what you actually want to do, mesh well with your champion's kit, and are effective against the enemy's champions and builds. Goodbye constant cookie cutter builds, hello having to think occasionally.