Lessee, what have I done to my players lately...
1) Darkmantles. The group of players I play with have a natural nemesis, known as the piddly little 1 hit die Darkmantle. In 3.0, back when darkness was, well, DARK, I'd spring a swarm of these on the party, and NO ONE EVER carried anything more powerful than a sunrod or a light spell; as a result, you have a bunch of adventurers stumbling around in the dark, guessing where the enemy was, missing concealment rolls, etc. ALL the while the blindsensing darkmantles were grabbing and constricting. Then the well-meaning PC's would keep hitting one another with missed strikes while trying to get the creatures off of the grappled members of the party. I could challenge them with groups of 4 or 5 darkmantles well into 8th level.
Nowadays, the weakening of darkness has put the Darkmantles squarely back at CR 1 or 1/2.
2) Energy Draining. Eric reminded me that energy draining undead, even with weakening of energy drain, can still scare a party. I had a party of 7 8th level PCs somewhat nerve-wracked because of a group of wights they faced. A few people got hit for about 3 negative levels each, and until the artificer could get his infusions back the next morning, they were on pins and needles, watching the clock tick down to 24 hours! Fortunately, the artificer fixed them up in time, but it taught one particularly brash cleric to be more careful stirring up trouble when the party is otherwise occupied.
(There's also nothing more amusing than watching a cleric beg an artificer for healing.)