Blueprints being usable from zoomed out map view is amazing, really makes remote building at scale possible.
Of course that could get really annoying trying to position things correctly, but the new feature of
locking blueprints to their own grid makes that trivial.
Placing down the blueprints for a big rail system and fitting things in correctly has turned from something that is slowly done over hours to something that takes seconds. You still have to build the entities but your bots can do that for some kinds of rail designs with roboports included. Make factory modules for anything and just add them in as you need.
Train stations can have a limit for how many trains are allowed in. Makes it possible to name all stops for same resource the same and let all those trains have the same schedule. It's so much easier to manage train schedules, mods like LTN are no longer necessary to avoid having to spend all your time giving individual names to each train stop. Really is a massive game changer if you use trains a lot.
And I love
the new blueprint library 
Description fields, hierarchical blueprint books, upgrade planners on whole books, blueprints usable without having them in your inventory and always available across saves etc makes planning factories just less hassle overall in many ways.
The new /editor mode together with mods like Editor Extensions makes testing things much easier. Currently using /editor mode to pause the game time while inspecting combinator creations and single-tick stepping through them to debug them. The best upgrade for making combinators more usable ever (together with Picker Dollies mod that lets you move things around without breaking wire connections).
Spidertron is really cool. It's better and more useful than expected from the hints long time ago. Artillery helps and transforms the late game more with how it completely can prevent natives from bothering you again, but Spidertron is a cool toy to play with with many features. It's like a combination of a remote controllable player (including personal roboport and requests) and can be used for combat in a way that is something in between going out yourself against the natives and using artillery.