I like(d) the move to having all character classes use a similar resource recovery schedule in 4E, and disliked losing that in 5E, it solved so many problems in balance and pacing I used to experience in 3E, and it enabled a relatively flexible way to run my campaigns.
I like Warls, e.g. Warlords and Warlocks. They both fit a thematic niches I really enjoy. A character like Roy from OotS was made with 3E in mind, but in truth, the 3E Fighter couldn't really play a good inspiring or tactical leader mechanically. He was only got at hitting enemies with weapons, and maybe doing some of the few combat maneuvers with them, and it would become rather samey (you got Improved Trip, trip with your last iterative attack, no real choice her.). And the Warlock, as someone making a deal with supernatural beings to get access to magical power is just great.
I liked that they got rid of the really bad save or die effects and overall lessened their impact. I've lost too many characters to stupid save or die effects like the 3E Bodak, even with excellent saving throws...