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.

Oct
How do you get your sac fragments in Path of Exile?
If we're talking about day 1 atziri farming, you'll be mapping enough to reach a decent level ~75-80 and you shouldve collected a few sets with whatever vaal Path of Exile Orbs you have

Jan
How to do Righteous Troll build in Path of Exile 2.5.x Patch
Chieftain with 9 endurances would have 13.5% regen not 15%. While chieftain would have more regen and probably a bit more damage, but he wouldn't be as tanky against physical damage.

Sep
Path of Exile Overview on Poe4orbs.com
Path of Exile is an online Action RPG set in the dark fantasy world of Wraeclast. It is designed around a strong online item economy, deep character customisation, competitive PvP and ladder races.

Oct
PoE Useful Guides - How to increase the experience gained
In the Path of Exile, the level is usually gradually increased during the mapping. This guide will show you how to gain experience more efficiently.

Oct
Path of Exile Useful Tips 02 - Check Your Unfinished Map
In order to help players better enjoy the fun of Path of Exile, we decided to share some useful tips here regularly. This time we will show players how to check your unfinished map.

Oct
Five New Sextant Mods in Path of Exile Content Update 2.4.1
Path of Exile Content Update 2.4.1 is scheduled for this coming Friday (New Zealand time).changes to the Atlas and new Sextant Mods!