I use some of the "gritty" rules from the DMG, the main one being a short rest is overnight and a long rest is (usually) a week or more. Then I throw 5-10 encounters with no long rest. So basically by the time they get to the last encounter or two they're low on HD (if not HP), spell slots and resources in general.
That and I'm stingy. Not a lot of gold, not a lot of magic items. People have to rely more on creative thinking and class abilities. I also limit raise dead and plane shift type spells (including banish). I effectively ban resurrection, teleport (except for line of sight).
Beyond that, there's always focusing fire, double taps, creatures attacking in magical darkness, using environment and things like I mentioned in the other thread about dragging people off. Oh, and being significantly far away from the party is a great opportunity for a flanking maneuver so the archers aren't nearly as safe as they think.
But ... I always discuss how deadly the group wants a game to be at the start of the campaign and they don't want a high body count. On the other hand, we never had a high body count when playing 1E either even though I can't really remember how we avoided it. So I'm not sure the gritty/high death count is the only thing. I wouldn't have problems killing off PCs if I really wanted to, you can always turn the threat level up to 11.
I also do a fair number of custom monsters and create "special" legendary or near-legendary monsters so people don't know what they're hitting. For that matter, even reskinning creatures with minor modifications seems to help with the "oh, this is a ___" issue. Lately I've been doing a lot of aberration type creatures and special conditions just to mess with people.
But I'm not sure anything can bring back that feeling of discovery, and there's always the option of just taking a sabbatical for a bit. It will still be here waiting when you're ready to come back.