Hit Dice: Monks have d8 hit dice, which makes it hard for them to function as a front-line fighter without support. Having high AC and other protection abilities is crucial.
Weapon and Armor Proficiencies: No armor, no shields, and proficiency with a mere handful of weapons makes monks one of the least useful martial classes until you consider their natural combat abilities. Your unarmed attacks will steadily increase in damage die but start at 1d4. At low levels you’re going to deal sub-par damage but will eventually grow to surpass most other classes with your sheer number of attacks.
Tool Proficiencies: Monks don’t gain any tool proficiencies.
Saving Throws: Strength saves don’t come up much, but Dexterity saves are one of the most common. Eventually you’ll get Diamond Body, which grants you proficiency in all saves.
Unarmored Defense: Your main goal as a Monk is to get to 20 Dexterity and Wisdom through ability score improvements, feats, and magic items. If you can achieve that, you’ll have 20 AC, which matches a fully armored character wielding a shield.
Martial Arts: This is why you play a Monk. You’ll rely completely on your Dexterity for combat. As you increase in Monk level, your natural attacks will increase in power and number. You get two-weapon fighting for free and are not reliant on weapons or fighting styles.
Ki: The Monk’s Ki reserve scales with level and is used to fuel a variety of useful abilities. Flurry of Blows is the most basic, granting Monks the most attacks of any class starting at second level. Additionally, access to Dodge and Disengage gives you more options for positioning in combat. This is especially important given your role as a secondary front-liner.
Unarmored Movement: Monks don’t have a lot of options for ranged combat, so the extra movement is important for ensuring you always have targets available.
Deflect Missiles: This only works against ranged weapon attacks. Spells are unaffected. A caster with a cantrip can slide through your defenses. Also, once you are high enough level that your enemies have multiattack you can only stop one projectile per round using your reaction.
Slow Fall: This will save your life and can be used to dramatic effect quite frequently.
Extra Attack: You can attack four times per round when you use flurry of blows as your bonus action.
Stunning Strike: You can use this any number of times in a round, so with flurry of blows you have up to four chances.
Ki-Empowered Strikes: This is especially important since you aren’t proficient with many weapons.
Evasion: With Dexterity as one of your main stats, you’re going to take little to no damage from effects that require Dexterity saves.
Stillness of Mind: This is very situational but being reliably on your party’s side means they won’t have to worry about how to deal with you if you get mind controlled.
Purity of Body: Disease and poison can really ruin your day.
Tongue of the Sun and Moon: This isn’t very important if you have a Bard or Wizard who can speak may languages or have access to Tongues.
Diamond Soul: Proficiency in all saves makes you more resistant to magical assault.
Timeless Body: Age almost never matters in game.
Empty Body: Invisibility is great, astral projection is neat, but generally not useful on a daily basis.
Perfect Self: This ensures you always have fuel for your Flurry of Blows whenever you get into a fight.