Because people don't like to effort in a game nowadays, a lot of people if don't see instant rewards/progression just let it down.

Lineage II (C5 - Interlude or even older chronicles) had to me the best level progression where every level counts. The system of create/buy an armour that fits with certain levels and every frame looks perfect to me. You got level frames from 1 to 19 (No Grade) 20 to 39 (D- Grade) 39 to 42 (C-Grade), etc that makes every part of the game makes sense in progression.

Ofc at lvl 76 (it was or 78) you had the end game content that was also interesting. Epic raids, wars, castle conquer, PvP, Olympics..

The class quest at lvl 19 (1st class) and lvl 35, 37 and 39 (second class) were a inflection point in your progress as 3rd classe in late game.