Spells and Items
You will exclude a few edge cases, though it is arguable whether or not these could be used in an opportunity attack anyway.
- The dismissal option of Dispel Evil and Good.
- Using Flame Blade, Mordenkainen's Sword, or Vampiric Touch to attack.
This will have a major effect on some monsters, who will be unable to make any attacks of opportunity because their only Actions are melee spell attacks.
For example: Specters won't be able to use Life Drain and Will o' Wisps won't be able to use Shock.
Monsters can always attempt an unarmed attack, but it might not be effective. For example, a Flame Skull with its negative Strength modifier does 0 HP damage. :-) Hat-tip to Medix2.
Not Allowed Anyway
Using the weapon created by a Spiritual Weapon spell or the whip form of a Quaal's Feather Token magical item normally requires a Bonus Action to make a melee spell attack. Rules designer Jeremy Crawford unofficially clarified on Twitter that the Spiritual Weapon is not intended to threaten or to be able to make Opportunity Attacks. Given the same phrasing, you could argue the same for the magical item.
The Sage Advice Compendium says that opportunity attacks do not allow you to cast a spell, so instantaneous spells that use a melee spell attack are definitely not allowed.
Per the Sage Advice Compendium, page 12:
Can you use a melee spell attack to make an opportunity attack? You can’t if the spell attack is created by casting a spell.
That Sage Advice section also calls out monsters like specter and banshee, whose only actions are a melee spell attack.
Grappling and shoving are not allowed as part of an opportunity attack.