Oh, 5e monsters need a ton of help. some of them are fine, like beholders, but most of them are the "bag of hp with some amount of hurting numbers" that are kinda boring after about 2 rounds. What I love about the legendary action idea though is the ability to add interesting new quirks to a monster off turn that can tell some of its story, like a prowling beast moving at the end of someone's turn to set up for a pounce, or having an enchanter controlling pawns. Those little actions that 4e uses immediate actions for, but since you get three a round, it lets you do smaller more nuanced things for that enemy. I tend to like movement abilities in that slot a lot, to keep fights from devolving into the stand still and hit guys thing that most 5e fights become, as without that, it is mostly disincentivized for characters to do more than set up shop and start hacking away or chucking fireballs.
I won't say some 4e parties don't have that problem, a good fighter or warden wants people to stay exactly where they are. but most classes and monsters gain something from changing position and there is so much forced movement, it almost requires those move actions. I was so excited to see that 5e was finally making movement a pool to be use through the turn, as that allowed for dramatic hit and run tactics, but honestly, only the rogue and monk really have any use for that without some real brain bending. I hate fights that feel like an old final fantasy or dragon quest fight where we all stand around and throw numbers at each other, until someone's numbers hit 0. That is why I loved using all kinds of terrain in 4e, and my most memorable encounters in 5e involved cool environments, since all the bad guys just move somewhere and hit the attack button.
