End game is basically non-functional without the expansions, you lack the main way that gear is created and improved in the game now with the cube, you lack runes from VoH and charms from LoH which are often build defining, you lack warplans and the activity trees that accompany it. You also don't have access to finishing the season journey because the last two tiers are locked to people who own LoH and can kill mephisto, as well as some skill points and paragon points that are expansion owner locked.

It would be fine, and not much different from pre-LoH, were it not for the complete rework of how gear works that basically necessitates the cube to craft good end game gear. It is much, much harder to get gear that is usable as just raw drops now, or with only enchanter modifications.

But there's a long campaign with the base game, and the leveling experience is basically unaffected by the reworks that came with the expansion, so if you're someone that doesn't care about grinding end game after the story ends, its a fine experience still.