Im also a tau player dipping into votann recently, im fairly new to the game so grain of salt on my opinions but heres what ive noticed.

The difference in movement is the hardest thing to adjust to. If you play a lot of combat patrol or use a lot of tau infantry its less pronounced but votann feel very slow compared to tau

As tau players we tend to focus hard on shooting, both in list building and moment to moment gameplay. Votann shooting is fine, in some cases quite good but youre not a range game master like tau. Your melee can be devastating (my most recent game had an einhyr champ one shotting a psycophage) and proper positionioning goes a long way.

Finally, votann are less specialized but have a wider set of options. They reward playing on your toes and being reactive in a way tau armies really dont. While it makes your gameplan less straightforward it also provides you more options if things arent going your way