Well, see my last paragraph. Like the last time I ran a shambling mound, it had a bunch of will-o-wisps with it that kept healing it with lightning (I believe this is a common combo, and I highly recommend it, it was fun).

The encounter was still interesting, even though I demonstrated the lightning interaction for them, and the one player who already knew about it didn’t have to worry about using his metagame knowledge. In fact, I’d argue it was far more interesting and dynamic than if I had based the encounter around not knowing about the lightning healing.

Sure, it’s fun when you can get new players with those surprise abilities, but that only works once. The more you play, the more experienced your entire table will get, and the less those encounters will work. So IMO it’s better to learn how to create encounters that don’t rely on surprise monster features in the first place.