New units

[Nirual]

there are frost spiders and ice dragons, both neutral and obviously kinda related in theming.
then there is the Lava Golem, which can only be summoned by a valkyrie skill so they kinda count as viking units despite being neutral.

Rune Mages arent new but changed considerably to work with the viking runes aswell as unused hero runes.

[ckdamascus]

Almost every race/unit has a subtle difference.

Archdemons can do AoE attack now with 4 reload, all demons release rage balls when killing, they also get a bonus if they hit with a critical...
Dwarves like to retaliate harder (15% bonus to person who hit them last)
Humans/Elves work better in familiar terrain (whatever terrain you have fought the most in, bonus to attack/defense stats)

A few other small details...

[camelotcrusade]

What CK said. You will find a lot of tweaks and ehancements to units across the board, so it's fun to discover them all and play around with it, I think.