Much to my annoyance, you can still use any creature larger than you as a mount (and don't have to control them). And they did not change the forced dismounting rules (any push/prone effect on either of you makes the rider do a DC10 Dex save or fall off the mount, prone)... even though there are a lot more push/prone effects in the game now (besides weapon masteries, so many regular beasts prone on hit now, no save).
What this means is that armored knights are the absolute worst at using mounts - it should just be Halfling Rogues riding the Paladin instead.
Then, your Paladin mount's Protection fighting style now gives disadvantage on all attacks against a target.
You're a Rogue? Pick Sentinel, get an extra sneak attack if they try to hit your mount over disadv-attacking you.
You're a warrior? Pick shield+Protection yourself, attacking either you or your mount is at a disadvantage.
(and if both of you warriors have PAM, anyone approaching triggers both of you at the same time)
etc
Technically you don't need to ride them to benefit from these, but it makes things so much simpler when you're always in the same square (your 'mount' can move without you worrying that they'll step out of Protection's range, it's harder to push you out of Protection's range, same PAM trigger). The only hitch is that uncontrolled mounts keep their own initiative, so that may sometimes mess things up, but someone on the team picking Alert / you just dismounting and fighting from the ground a few steps away when needed isn't the end of the world.