95% of builds on Path of Exile scale with special uniques or powerful rares
The reason it is still fun after 20+ hours of play is because of the skill and gear progression that comes with most builds in the game. 95% of builds on PoE scale with special uniques or powerful rares as well as getting gear that has more linked sockets (5 socketed and linked sockets on gear are a lot more rare than 4 linked gear, and 6 linked gear extremely rare and endgame to acheive in most cases). Both of these will change will change how the skill works/increase their damage output significantly, making you want to progress even more to see how far you can take a build idea.
So the long and short of it is yes in it's most basic form the fun of playing the same skill over a long period is to see the returns of your investment into the skill and seeing how far you can take it, which is much more rewarding than it sounds due to the sheer complexity and number of options the game has to offer.

Dec
I still have so much fun in PoE after so many years
The beginning is the best thing ever! My tips for you is bind your left click button to Move only.

Dec
All that the jewel sockets and Clayshapers are a pretty big investment
getting jewels will be hard, and without them you cant really play arround Golems, unless you go normal summoner first then switch.

Feb
poe4orbs:POE 3.17 Siege of the Atlas Official Trailer
Really think the new atlas skill tree is going to be a success, personally i've played PoE for a long time myself, and looking just slightly on this, i really think it just looks fearsome yet it looks really nice developed so newer players can easily look where they want to target farm. Especially comparing it to the old version where a lot off new players never even got to the point to actually start target grinding, well done!

Dec
Real 10% OFF for POE Chaos and Exalted Orbs in Xmas and New Year
How time flies! Christmas is coming again. Guess you guys cannot even wait to celebrate and enjoy this wonderful festival! And we have also prepare gift for all of you!

Dec
Improve your clear speed and also make the PoE more enjoyable
Just 2 quick things that might improve your clear speed and also make the game more enjoyable

Jan
PoE New Expansion Review: Echoes of the Atlas
Echoes of the Atlas is the latest large expansion of Path of Exile, with a newly designed endgame content and reworked ascendancy classes. In addition, there are 11 added maps and new currency rewards.