My guide to reading a 401k plan document, which most people never look at.
The most important things to find:
The fund list and expense ratios: look for total expense ratios (net expense ratio) for each fund. If the cheapest equity index fund has an expense ratio above 0.15%, that's worth noting — you may want to use an IRA for some contributions instead.
The vesting schedule for employer match: cliff vesting means you get none of the match if you leave before a certain date, then all of it. Graded vesting phases in over time. This affects the real cost of leaving a job before you're fully vested.
After-tax contribution option: many plans allow after-tax contributions beyond the $23,000 pretax limit. These can be converted to Roth within the plan or rolled out to a Roth IRA — this is the mega backdoor Roth. Check if your plan allows in-service withdrawals or in-plan conversions.
Hardship withdrawal and loan provisions: hopefully you never need these, but know they exist and what the terms are.