By itself, Mark is a slight buff to the party's melee combatants (but also to the GM's melee combatants), and a slight annoyance to ranged combatants. Having advantage on opportunity attacks makes being in close quarters more of a threat, and increases the necessity of the Disengage action, especially for characters who don't want to be in melee range. It could be considered a higher value for the DM than for the party if combat encounters tend to have a much larger number of enemies - you would be able to have several enemies "lock down" the party's movement, which would force them to either continuously Disengage and use their bonus actions to attempt to fight off their enemies, or be locked into melee (especially if there isn't a difference in speed between the PC in question and the pursuing enemy). It forces the party (and you) to more carefully consider their positioning in combat - if that's the kind of thing your players enjoy, it could be a great addition. If you do introduce it into your game, I would make sure the party knows about it before any combat occurs, so that both sides are able to use it to their advantage.
Where Mark starts to shine is when it is combined with certain feats and combat abilities. While Charger cannot be used to create opportunity attacks, it does allow characters to re-position enemies, interrupting the enemy's ability to use its Mark. If a Fighter (or another melee class) uses Mark in combination with Sentinel, they gain advantage on the opportunity attack (which they are allowed to make even if the opponent disengages) and if they successfully hit the enemy, it is forced to remain in close combat (where the Sentinel Fighter can attack it again during their next turn, and renew the Mark. Polearm Master is also worth a mention, as it allows the Fighter to Mark and make opportunity attacks against creatures within 10' instead of just within 5'.
Lastly, while Mark does allow an Opportunity Attack without expending a reaction, without Multiattack / Extra Attack, this doesn't increase the total amount of OA's by more than one - combatants now get one OA against a target they attacked on their turn, and can use their reaction to make a different OA against a different target (provided one is available). Combatants are still limited to one OA per turn - meaning that unless they are in melee combat with two enemies, Marking is unlikely to increase their damage output. It does allow a combatant to preserve their reaction, which seems to benefit PC's slightly more than monsters, as they seem to have more reactions available to them.
