TL;DR Blizzard doesn't update Diablo 3 enough to keep people interested, so PoE GOGOGO
Hate to be one of those guys, but yeah, Path of Exile.
Like the problem with RoS for me was that, while RoS was a huge change from vanilla D3, it just wasn't enough to keep you going for long. It was good for like a month, yeah.. But what really changed with RoS? Numbers got bigger. You deal more damage/have more life, monsters deal more damage/have more life.. It's still the same amount of hits to kill a monster. Maybe you see some different graphics when you press your skill buttons, but it's all the same shit at the end of the day. And yeah the mystics brought in the endless quest for perfection, but the difference between say 98% CHD on your amulet and 100% isn't going to make or break your character.
But since launch, what has really changed with D3? I haven't kept up, but to my knowledge, the hot topic is still patch 2.x.. Which it was when I stopped playing roughly 3 months ago.
Path of Exile is fun because of the leagues. There's a set of (nearly) constantly going ladders, with a little downtime between. They were 4 month ladders, now they're moving to 3 month ladders. And we just had a one-month race. The first set of these ladders/leagues brought in rogue NPCs, that were pretty much little bosses that walked around and kicked peoples asses. It also brought in a mod called 'onslaught', which is used on some items. The next set of these leagues brought in Shrines, which while taken directly from D2, are cool as fuck regardless. It also brought in 'Nemesis', which are just a pool of more difficult than normal rare monster affixes. They're something to watch out for though - if you don't, they definitely can kill you. (Which is actually a negative in PoE (10% XP loss), not just a lol cya 5k gold RIP) After that, the next set of leagues brought in 'strongboxes', which are just like super-duper chests. They have basic themes - armor, weapons, jewelery, gems, etc.. That can spawn with, or have rolled onto them, mods which affect what they drop and what monsters come out of them. Typically, it's hit a box, a fuckload of mobs spawn around you, and you fight them. As with everything in PoE, there's always some bullshit combination to look out for. (For example, if the chest freezes you when activated and has 3 additional rare (champion) packs guarding it, you can get fucked pretty quickly if you don't have a way to thaw your ass out quick) And it also brought in Invaders, which are just fucking off da railz asshole bosses that sometimes appear. Lots of 'oh fuck's were had on their behalf.
And those are just the updates from the leagues, let alone game patches. The game changes very, very often.. And that's what keeps it fresh. Unlike Blizzard, GGG has their shit together and takes less than one year per content patch.
PoE also runs on an expansion system, I think they're aiming for 3 major expansions per year. So far this year, we've had Sacrifice of the Vaal, which is basically an introduction to a new currency item, which can either fuck up your gear or make it great, and a new endgame boss. With that, her like 8 different uniques, a line of Vaal skills, the currency item, (http://pathofexile.gamepedia.com/Vaal_Orb) and the leagues that launched at the same time. With the next coming leagues (Just under 2 weeks) we'll be seeing a Mystic-like feature, and a system like Diablo 3's massacre bonuses. Instead of a massacre bonus, you'll earn a 'killstreak' (CoD ripoff obv shit game 0/10) reward per X monsters killed without 5 seconds of downtime, growing in power each time. It's shit like X% Movement speed, then X% attack speed, then some crazy powerful shit I.E. corpse explosions going off everywhere, all this crazy shit.
And in that time, Diablo.. Nerfed the furance, I guess. PoE isn't perfect, but as someone who played a lot of Diablo, it's fucking miles ahead of Diablo 3. And it'll stay that way, due to the nature of their updates. The core of the game changes every 4 months, ontop of regular content patches. If you're not a ladder player, there is a standard league that has no time limit on it. Even there, the economy is far from stale.