It can sometimes be beneficial to choose a class like Fighter or Rogue at level 1 for their excellent martial/skill proficiencies, even if they aren't going to be your main class focus.

Proficiencies: Classes gain fewer weapon, skill, and saving throw proficiencies when chosen later on than at level 1.

A Wizard 4/Cleric 3 can only cast level 2 spells from each class, but will have a large number of spell slots to devote to each, with their Cleric spells able to go in Wizard slots and vice versa⁠-it's not known if Larian has tweaked that progression from tabletop rules, though.

Additionally, spellcaster/spellcaster multiclasses have a shared pool of spell slots, removing a major disadvantage of this kind of character in older versions of D&D.

Spell levels: Full casters gain new spell levels and spell slots every odd class level.

Most spellcasters, and also Paladins, choose their subclass at level 1. This is often, but not always, the minimum number of levels you want to hit in a second or third class.

Subclass: Most classes let you choose a subclass at level 3 (your Bard college, for example).

Extra Attack is a priority ability you don't want to delay more than one or two levels.

Extra Attack: Fighters, Barbarians, Rangers, Monks, and Paladins get a second full attack per turn at level 5.

Diablo 2 assassin builds