I wish employers would do a better job of explaining how the impact on your paycheck isn't dollar for dollar. Too many people think that contributing 5% to the 401K means losing exactly 5% of their take home pay, which isn't true. The impact is much less and employers/plan managers don't do a good job of debunking that myth, either for 401K's or HSA's.
I, too, am amazed at how much money people leave on the table. At DH's work, they match the first $400 of HSA contributions, dollar for dollar. They also give $2k per year into the HSA if you participate in the wellness program and earn a certain amount of points (which are super easy to get). And there's the 5% 401k match and the profit sharing, which is deposited directly into the 401K every year, but only if you participate (and it need only be $1 per pay period into the plan to get the bonus). But so many people don't participate in ANY of this. It's an awesome program and we grab as much $$ as we can every year. But so many people either can't be bothered to do the (minimum) work it takes to sign up and get points, or they think the impact on their take home pay is going to be too great. It's a shame.