Welcome to the vanilla SMP.
There have been a couple changes made to the server for a more customizable and harder experience.
The big changes that have been made are...

Ranking up and mob levels
When you begin, you will start off with a beginner rank and can rank up by getting experience points in the game.
Higher ranks give you more commands to use in the SMP like /hat. To check your rank and progress to the next rank, do /rankinfo. To rankup, do /rankup.
Aggressive mobs will now rank up with you, and their current health and level is displayed on them.
High levelled mobs deal more damage, have increased HP, and other increased effects like speed/knockback but now have a chance to have increased drops based on the level.

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