The Delve League started last August and will end in November. It introduced an endless dungeon in the form of the Azurite Mines. Players can delve as deep and as far as they want, collecting sweet loot and PoE currency along the way. They can also gather Azurite and VoltaxicSulphite to improve their equipment and items. The latter is for upgrading the Crawler, which guides players through the mysterious darkness that took over the mines.
The Reason for Delve
The idea was to keep players from dropping the game after completing the current league. An endless dungeon screams “complete me”. However, it's not going to be completely explored at all. It's in the name. Players would have to go back, try to improve their skills and equipment, just to reach the deepest level they can get to. Testing out your skills and reaching the most difficult levels, after all, make games worth playing.
The Problems
As it is with other updates, the Delve League also has its own flaws. For one, the VoltaxicSulphite(what the Crawler uses for fuel) nodes you get to gather are quite scarce. Without it, delving further would be impossible for players. Hence, they are forced to stop to collect the fuel elsewhere. It's jarring especially when you've finally reached your stride and are forced to an abrupt stop. Even if the dungeon is infinite, you can't explore it continuously.
Another constant problem is that too much delving saturates the PoE trading market with items from the mine. It's not much of a problem right now, though, because it's the current league mechanic. However, it would definitely be a different story if it goes core. For the uninitiated, that means making the current mechanics permanent.
Now, you may ask, ‘why is that a problem?'. Delve loot can devalue the future league's loot. They're going to be cheaper and easier to collect than loot from the current (at that point) league. That would cause players to be quite reluctant when challenging future mechanics.
There are more issues such as bugs, and eventually, it tends to become a grind. In general, though, players seem to be refreshed by the change of pace the Delve League brings.
The Solutions
If Delve is indeed going core, the developers should fix incessant issues that may potentially cause trouble for players—especially the loot problem. As it is, unique equipment currently seems a tad bit too common while delving. There are also complaints about how difficult it is to identify destructible walls and the fact that the Crawler moves independently of the player.
If it does end up in the core league, it could be an alternative to endgame or leveling. Otherwise, it could be an optional activity, a break from story quests, or something to do that's different from the usual. At any rate, the only thing we can do is wait for the official announcement. There's still about a month left before it ends, so continue enjoying Delve and Path of Exile!