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

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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.

Grief prevention
In order to keep your loot and base safe, you can now claim a portion of land which can only be edited by you unless you give permission.
To claim land, obtain a gold shovel and right click one corner of the land, then go to the opposite corner and right click the other corner.
To delete your claim, do /abandonclaim while standing in the claim. To allow other players to access items and build in your claim, do /accesstrust .

Inventory and Chest sorting
So you do not lose your mind searching for that 1 item, you can now sort your inventory and chests via /chestsort.
You can set if you want to sort your inventory/chests automatically or on a key press.
I would recommend enabling empty and fill chest so you can quickly dump items into chests without leaving a mess.