Spreading too thin
The most common mistake is running four damage skills with no supports. Each one does a fraction of the damage it could.
One skill with three supports does more than four skills with none, and it costs fewer sockets. Pick one.
Ignoring resistances
Resistance is capped, and the cap is what keeps you alive. A rare item with higher armour but lower resistance is a downgrade, no matter what the item level says.
Check the panel after every gear change. This one habit removes more deaths than any other.
Chasing perfect rolls while levelling
A modifier at 80% of maximum is fine at level 30. Spending a rare orb to close that gap is spending endgame currency on an item you will vendor.
Use the cheap orbs and move on.
Buying gear instead of support gems
The cheapest power in Path of Exile 2 is a support gem for a skill you already use. It multiplies what you have, and it costs a socket rather than currency.
Check your supports before you look at the market.
Following a build guide completely
Build guides assume a level and a stash. Following the gear plan early leaves you unable to afford the passive route you committed to.
Follow the passive tree. Ignore the rest until you are there.
Pushing forward underlevelled
If enemies are taking longer to kill than they did two levels ago, you are behind. Pushing forward makes it worse, because the gap compounds.
Go back and clear an area you skipped.
Skipping the trial content
The trials feed ascendancy, and ascendancy is the largest upgrade your build gets. Players who wait for endgame delay their own build by twenty levels.
Run the Trial of the Sekhemas as soon as the level requirement allows.
Not using a movement skill
Anything that closes distance or escapes a pack reduces deaths more than any stat you could take instead. If your skill bar has no movement skill on it, that is the first thing to fix.
The habit that fixes most of this
After every gear change, do two checks: is my resistance capped, and does my main skill have a support it could use.
Two questions, ten seconds, and they cover the majority of what goes wrong.