Not there just yet, but will soon be starting Ultimate for the first time 
Any general advice for players making that transition? What are the big differences other than ‘harder’.
Remember there’s more reduction of your resistances, so make sure you have good resists…
Other than harder? I’d say difficult. 
In general ultimate isn’t really as different. You cap your resists and armor absorption and you’ll be (mostly) fine. At the first glance it’ll be hell for you, but when you learn the difficulty (after act 1 or 2) you’ll stop being afraid and will even forget you’re playing something harder. You have to remember enemies’ damage is raised drastically in ultimate, so facetanking bosses might be a terrible idea sometimes. I learned it the hard way, I was sure I am tanky because I have a shield and I chose Soldier mastery. Nope. Just nope. :rolleyes:
Oh great! My burn based Mage Hunter smashes everything atm. Guess I’m in for a surprise then :-/