The page 163 of the Player's handbook says:
However, even they are an optional rule, feats and multiclasses are allowed in the Adventure League's plays, and some claim they as a core part of the edition. Thus, I want to know how was your experiences without theses optional set of rules. This include gaming balance, martials against spellcasters, fighters without feats and so on.
Thanks in advance.
Game works fine without Feats & Multiclassing. Multiclassing favours casters while Feats favour non-casters, but it's not a huge difference. More magic items, especially weapons, easily compensates for any imbalance caused by a lack of Feats. Feats can help compensate for martials being underpowered in a game where there are only 1-3 encounters per Long Rest; IMCs with 1 week long rests it's common to see 6-8 encounters per LR, and the balance is fine.
No Feats means simpler PCs, faster combat, and makes it easier to have PC-class NPCs. This reduces the PC-NPC divide and makes PCs feel more embedded in the world.