Well, a lot depends on your character build, but...
Barbarian 2 = Rage (for 1/2 damage and adv on STR checks), Rage Damage, Unarmored Defense, Reckless Attack, and Danger Sense (good for those poor DEX saves)
Fighter 2 = Fighting Style, Second Wind, and Action Surge (all short rest abilities)
Monk 2 = Unarmored Defense, Martial Arts, Ki (very useful and short rest also), and Unarmored Movement
Paladin 2 = Divine Sense (meh), Lay On Hands, Fighting Style, Spellcasting, and Divine Smite
Ranger 2 = Favored Enemy, Natural Explorer, Fighting Style, and Spellcasting
Rogue 2 = Expertise, Sneak Attack, Thieves' Cant, and Cunning Action
2 levels of spellcasters will get you a lot as well (most include subclass features), but hopefully you get the idea.
Depending on your build, the second Fighting Style can be really good, too.
I am not saying Extra Attack isn't powerful, I am saying it is. However, all the features you gain from 2 levels in any battler class can make up for that power and make doing the MC dip worth it. After all, you gain a lot for delaying Extra Attack a couple levels.
The real downside for doing the MC dip is losing out on the early ASI/feat choice.