5e doesn't have detailed rules telling you exactly what you can do and when exactly when you're allowed to do it. This allows for far more complex combat than 3.5/3.75 ever allowed.
Imagine that you're on a raised part of a ship deck playing a pirates campaign. There is a rope loosely "tied" to a pole. You want to do what they do in pirate movies. Grab on to the rope (use an object), swing (use movement), attack, keep swinging (use rest of movement), then let go of rope (no action required). In 3.5/3.75, you'd be told you can't do that because the rules force a specific way of playing. At my table, I'd give you an advantage on the roll if the target is currently in combat with someone else.
In 5e, you're limited by your imagination. Some people lack imagination and would prefer to be told what they are allowed to do, thus they complain that 5e is too "simplistic".