Starting from the top, AFAIK
OOVEE the publisher for original spintires had a falling out with dev. OOVEE have been kicked to the wind. So hopefully game development will continue.
More maps, more cars, new sandbox map (whatever that entails) challenge game modes.
More physics simulations. E.g. fuel in tanks now have fluidic movement changing balance.
Unconfirmed but from dev stream says they are also looking to improve various aspects that the general public commented on. I.e. better camera angles, a solution to users who use steering wheels with respect to view (maybe a cockpit view) but they said they were looking into these.
Biggest thing is that mudrunner is open to consoles now.
Think of it as more of a 1.5 update. That is why you get at the moment 55% off if you have the original. That means with the dodgy publisher gone and hopefully with the new ones on board, the game can continue to improve and grow.
My personal opinion is that for the extra content, thats not a bad price. Especially as its wide open to modding so the usual variety of maps and vehicles.