The hardest adjustment will be in the areas of patience and discipline. Being card dead or running bad in a live game can be extremely frustrating due to the decrease in hands per hour compared to online games. Downswings can feel eternal.
You'll watch horrible players scoop pot after pot while you fold rags for hours on end or run into cooler after cooler. I'm not saying this doesn't happen online, but when you get double (or more) hands per hour compared to live, it's easier to mentally endure. If you are a multitabler online, the adjustment will be even more difficult as you will go from hundreds of hands per hour down to about thirty (or less).
The aspect of physical tells
also comes into play in a live game. Not only will you need to learn to pick up on other players' tells, but you'll need to hide your own from experienced players.
Good luck at the table!