Splendor is the gateway Euro every veteran hands to a new player on day one. The teach takes five minutes; the strategy goes deep enough to reward 20+ plays.
The single biggest tactical decision is whether to chase nobles or chase points. Nobles (the visible aristocrat cards) give 3 free points each but require specific gem-colour balances. A player who can claim two nobles wins roughly 75% of games — but committing to noble requirements early forces you to buy cards in colours you might not otherwise want.
Reserving cards is the misunderstood mechanic. New players reserve cards they want to buy; veterans reserve cards they want to DENY opponents. A card you reserve is unavailable to anyone else, and you get a gold (wild) token in return.
The two-player game is its own beast. Tempo and denial dominate; the optimal play involves more reservations than purchases through the mid-game. Three-player is the most-cited sweet spot.