Opportunity attacks, indeed, reaction-type actions in general, are an example of a mechanic that is good on paper, and...not so good in practice.
If you like attacks of opportunity, well, no offense...
Apologies aside, I have my reasons for removing reactions from any game I put together.
First, they are hard to use well. Players with only one reaction a round have to choose which threat to react to before they know all the threats. If we are aiming to get players to strategize like it was real combat (not necessarily simulate real combat, but give players the thrill of real combat) then asking them to make a decision out of game that could not possibly be taken in game not only only interrupts the thrill and pace of the fight, but also detracts from the immersion.
Second, it encourages metagame strategy. A la "If I move now, and provoke an AoP, then my friend can escape without provoking an attack." This doesn't just affect the players, either. The DM is caged in by the players's near infinite ability to instantly react with sword swings.
Against a single dragon? If the dragon moves away, the party essentially gets a second round of attacks.
4th edition Dungeons and Dragons attempted to fix this with the defender archetype combatants, which could place penalties on people who tried attacking the squishy/fleeing allies or getting away from the defenders themselves. Other instances include spring attack and fly-by attack feats in 3rd, among others.
But this doesn't jive with sound game design philosophy: creating an exception to a rule, which is already an exception to a basic rule of the game is inconsistent and wastes space, time, energy, money, ink, etc.. Why give everyone the ability to attack out of turn just to allow a quarter of the combatants (if not more) to deny that ability?
Here's the upshot: First, don't give the players of your game an in game option, just to grant another player (or non-player character) the ability to strip them of that option easily.
Second, don't give players a choice that their characters should never truly have. A knight in melee with three orcs doesn't let one flee for fear of not being able to swing at the second one should it flee too.
And lastly, if you want the players to be able to "react" in combat, rather than just "act," then you should allow for players to pass, or "wait," and jump in on the action later.
Yes, this screws up many initiative systems. But if you make a simple rule to avoid abuse (such as -2 to all rolls taken in a reaction...after all, there's a price for not being proactive) then you will have real strategy occurring at the table, and you can ignore who isn't taking their all-too-important 5-foot steps.