Assuming you last played in 0.C and are now trying the experimentals, the first things I’d notice are the difference creation methods (multi-poo, single-pool, freeform), followed by encumbrance (basically, multiply the old system by 10 - it allows things to be small and not matter… until you get enough of them to matter).
The next thing to know is that there are more kinds of zombies, and over time, zombies evolve into the nastier versions, so, a year in, you’re going to be seeing a LOT more brutes and hulks - also, watch out for predator zombies, they’re NASTY. Necromancer zombies raise non-pulped zombies near them, and zombie masters insta-evolve nearby zombies to nastier versions.
As §k said, the wander spawn system is MUCH better. NPCs are somewhat better. There’s a new bionics system that limits how many bionics you can have, but I don’t think it’s in force yet. The vehicle system has been fiddled with a bunch, though some of that got rolled back recently - biggest change I can think of off hand is how jacks work (basically, there are “levels” of jack, and you need better ones the more the vehicle weighs).
