Main story seems to be that Zero has access to a higher tier of suppliers. Fast Ace suspension replaced with Showa components, Bosch ABS brake systems (rotors are J-Juan in front and rear, vs Nissin front and J-Juan rear in 2014). IRC tires replaced with Pirellis. New mirror design.
I think Zero finally has components that are appropriate for the price. Which is good, because every model was bumped upwards by $350.
The FX soldiers on with no changes other than the components above. The ZF2.8 modules are identical to 2014.
In addition to the component changes, the S/DS/SR bikes all have 10% more capacity for the non-power tank bikes. Like the 2010 => 2011 capacity increase, Zero is probably using a cell configuration with 10% more amp-hours at the same voltage. This add little weight - about 10 pounds - and range improves by about 10%. Not bad.
The largest Zero now has 94 miles of constant 70 mph range, and can ride for > 2 hours at constant 55 mph. Incremental gains over 2014, but the increase in range is essentially "free".
***
Some interesting comparisons:
The ZF8.5 is replaced with the ZF9.4. An interesting point of comparison is the 2012 S ZF9; the smallest 2015 Zero S now has more capacity than the largest 2012 Zero S. The 2015 bike weighs about 30 more pounds and has about the same city range, but component quality is much higher, it's much quicker (~10s 0-60 vs 4.8s), it charges more quickly, and highway-only range is about 30% higher.
Oh, and the 2015 bike costs $650 less than the 2012. Big big strides for Zero in a small period of time.
***
Unfortunately, the big loser is the charging. Zero still has no effective solution for high-power J1772 charging, though they continue to sell the $300 J1772 adapter. The expensive and limited compatibility CHAdeMO charging accessory is still available for the 2013-2014 bikes, but is not listed as compatible with the 2015 bikes.
Zero has clarified the number of accessory 1 kW chargers that can be slaved together to charge more quickly: the FX ZF2.8 can take 1 external charger, the FX ZF5.7 can take 3 external chargers, and all S/DS/SR bikes can take 4 external chargers. This is probably aimed at fleet sales; the Zero S bikes can add 52 miles of mixed riding with a 1 hour charge, allowing a police officer to return to home base for paperwork or lunch and pick up significant additional charge.
***
I hope hope hope that Zero has spent engineering time to make its 2015 bikes bulletproof from a reliability and durability standpoint, as well as time grooming and training their dealer network who are now going on their third year of this generation of bike. User forums will always over-represent failures, and I suspect the 2014s are more reliable than the 2013s, but I hope to see a marked reduction in errors for 2015.
***
So who's going to buy the 2015 bikes, and why?
* some 2012 and 2013 owners will upgrade, especially as their bikes fall out of warranty.
* higher-spec components will draw some who turned up their noses previously
* ABS is a big win for rider safety
* Zero is whittling away at single-charge range anxiety
* the smallest S bike is a straight upgrade for 2012 S ZF9 owners (like myself), and has very useful range for almost any rider
* the middle S bike offers most of the range of the 2014 S ZF11.4 + Power Tank model at a much lower cost/weight and while preserving the "tank bag"
* the largest S bike offers significant single-charge range, at significant cost
Who's not going to buy?
* Zero dropped their entry-level XU bike for 2014, and increases prices across the board for 2015. I think the overall value is higher than it's ever been, but there are effectively no routes to owning a new Zero that doesn't start at $12k.
* Expecting faster J1772 charging? Go straight to Harlan @ HollywoodElectrics and/or Brammo.
* Looking for faster DC Quick Charging? Try again in 2016; if the 2015 bikes are like the 2011, then perhaps the 2016 bikes will introduce a higher voltage battery and significant motor / controller changes.
* Styling changes minor, limited to wheels, mirrors, and the red rear shock.
* Anyone who can't sit on a bike at their local dealer probably isn't going to buy. It seems Zero has been signing up more dealers though; and the Harley Project Livewire tour has significantly raised awareness of electric bikes. $18k for the big-battery Zero S will buy a bike that's faster, cheaper, lighter, easier to ride, and (in some cases) has more range than the Harleys.