Let's be clear: I haven't done this as part of my job, but I have read multiple 401k (and 403b) plans cover to cover for various reasons.
(1) Make sure it's a safe harbor plan, if at all possible. This will - I think - require some matching % by the company. It should also help the company reduce compliance costs. For employees, this allows what would otherwise be highly compensated individuals (set by salary amount or # of people depending on various things) to contribute the maximum amount without various quirks and potential issues of refunded money for those individuals.
(2) In service withdrawals: you noted this, and I agree, but check both (a) how often they are allowed - preferably at least once/6 months, better if more frequent is allowed and (b) What is the fee, if any? Note also this means that the 401k has to hold after tax in a separate listing (not separate account, but they have to track sources to be able to do these correctly - some bad 401k providers don't do that)
(3) Contribution amounts: What % can be contributed before tax and after? My plans have had some strange limits, so think about this and how could someone with a reasonable salary still get very high toward the yearly limit? If the after tax is limited to 10% or 20%, it may be impossible to use that to maximum benefit. Why it isn't possible to set either before or after tax to 100% is beyond my understanding.
(4) How specifically does matching work? Some people want to do all before tax (and all after tax) at different points in the year, with the assumption that the after tax part is still matched. Also, if you hit the limit in September and don't do after tax (or don't do any) contribution after that point, is there a "true up match" [I think this is the "term of art" for the feature] later in the year to make it as if the person contributed the amount more evenly in the year (or do they just lose those dollars - and thus have to plan more carefully across the year)?
(5) How do bonus contributions work and how are they matched? This interacts with #4, and it can be VERY challenging to max our your 401k and get the max match if there is no true up and the bonus is received late in the year (my current situation is like this - argh)
(6) What can be transferred into the 401(k)? Can you bring in an IRA? Another 401(k)/403(b) [or really any 401(a) plan]? All should be possible.
(7) Include Roth 401(k)
(8) Eligibility options: Is there a delay to matching (bad) or delay to when you can first contribute (bad)? Any vesting period (like on a match) can also interact with in-service withdrawals in poor 401ks, so needs to be looked at/questions asked.
(9) Low cost options available (obviously not a problem with Vanguard, but needs to be included in an overall requirements list)
(10) Make sure the match occurs with the paycheck. Some companies will do a match on like Jan 1 of the following year, and that basically steals away potential growth from the employees under the guise of ensuring they get the full match. Better to have a "true up" if someone doesn't contribute evenly.
(11) Consciously consider if there is fee equalization. I am NOT a fan of this, but it's becoming more popular. IMHO this is not a great thing for folks into low cost funds.
IMHO Bonuses if:
(a) You can specify a precise dollar amount to contribute instead of just an integer percentage
(b) Auto-enrollment / auto-increase...but this gets into a very different thing than optimizing for early retirees.