All of my observations below relate to bounded accuracy:
This one is obvious now, but when I was first reading through the PHB I thought the various Conjure Minor Elementals/Animals/Woodland Creatures spells looked like trash. "8 CR 1/4 creatures? Pshaw, who'd want that." But then when you actually look at the CR 1/4 creatures, the spells are all actually pretty OP. I blame the spell less than the wonky CR system, but in any case the deadliness of CR 1/4 creatures is probably the biggest surprise of 5E. Don't get me started on wolves, cobras, and smoke mephits.
I've been a little surprised how effective poisoned weapons are. When my players first looted the poison off a dead drow patrol, I didn't fully appreciate how many monsters have poorish Con saves, and in practice a DC 13 Con target (applicable multiple times per round) is really good. Other poisons, like Purple Worm venom, are enormously effective compared to magic items.
Maybe I shouldn't have been, but I was surprised by how effective Pass Without Trace is in 5E. The combination of disadvantage on perception checks (in darkness of course) and +10 on Stealth makes even a clanking, heavily-armored Dex 11 Paladin actually pretty good at sneaking up on people, and therefore at getting surprise.
I've also been a little bit surprised how often Athletics is useful. It doesn't look that good on paper since it doesn't apply any directly-helpful effects, but it turns out to be top-notch at exploiting environmental conditions such as a vampire's vulnerability to sunlight, or a wizard's Web/Wall of Fire spells. Frequently Grapple or a Push is actually a better option than an attack--maybe not often enough to build a character around it, but often enough that I'm glad to have a Str-based melee paladin in the party. For this reason, Dex is less of a god-stat than it looks like on paper.
I've also been pleasantly surprised by how tanky Paladins are in this edition, especially with a splash of Sorcerer. The combination of heavy armor, Defense style, Shield of Faith, and Shield (when your hands are free) together with Bonus Action spells like Sanctuary and Quickened Mirror Image is top-notch for standing off hordes of enemies. I initially thought the party was pretty good with just a necromancer, bardlock, and shadow monk, but adding a paladin has been way more transformative than I expected even though he doesn't factor prominently in every fight. See also comments above about Grapple/Push.