Path of exile is not a bad action rpg, mechanics-wise and systems-wise, but some flaws in the design philosophy and the overall look and feel of the game won't make me want to play it anymore when it goes into open beta soon. Here are the things it does right and the ones it does wrong and how I would fix them up
My biggest problem with the game is responsiveness
Everything is floaty in path of exile, your inventory items seem to float around for a few seconds when you try to move them, same thing for other menus, of course this isn't game breaking but as you often find loot to equip often, it gets annoying to mess around with all these items, and boy are there tons of random items in Path of Exile. There's no money to be found but instead you trade usable items with various functions that can be as basic as identifying items or as complex as re-rolling numerical values on the modifiers your magical items have.
The biggest problem with this floatiness tho, is in the combat, the combat is really vague, imprecise and seems completely off half of the time. Opening containers weirdly where your character will move on top of the barrel or the rock then step on it in an awkward way, and you'll have to be exactly on top of it. Melee combat is the same, your moves seem to have no reach and you never know when you're hitting your enemies, you'll just stay there and slash at air and they take damage. Your melee special moves aren't better, they create animations that might hit enemies but there's no sense of anything connecting like in Diablo 3 or Torchlight or Titan Quest.
Magic spells and ranged attacks might be slightly better because you can see if your arrows hit your target and if the fireballs fall on the enemies but they still weirdly feel off, you don't get cool effects on enemies hit, they don't stagger much and they certainly don't fly away when they die.
How I would fix this
First of all, fix the interface floatiness, the game isn't that impressive graphically and shouldn't be too system intensive so tightening up how that feels would help part of it.
For the combat part of thing, having a wider range on melee attacks and more responsive animations would help greatly. Anything to show that your hits connected like damage numbers would also help me know if I'm hitting my targets or missing (because of the accuracy stat) or plainly missing (because I'm too far away)
The game doesn't feel ready yet
PoE tries many things to keep itself different from other games in the genre, first and foremost, you get a huge passive skill tree where you unlock nodes at each levels and they rank from passive stat bonuses to different abilities to ways that modify how your character works such as using life to cast spells instead of mana or applying your rechargeable shield to your minions instead of you. It's fairly overwhelming and it's not really a good way to present information since you have to find where you want to go and you don't know for sure where you want to go. It doesn't add anything except novelty.
How to fix this
Make it a talent tree of some kind, you probably have all these skills connected in similar ways but you would know what kind of path you needed to go down in.
The gem system is interesting, you find gems that make up all your abilities and they are divided into three kinds, strength, dexterity and intelligence gems. Some gems boost up other gems with various attributes (such as increased fire damage, splitting projectiles or raising the health of minions summoned). You get gems from quests and they can also drop from enemies. Also, the gems level up as you do and when they reach a certain amount of experience, you need to respect certain requirements such as level or stats to be able to increase their powers. It's an interesting idea, of course, but it goes wrong in my actual experience.
Either I'll never get any interesting gems (and it's the only way to get any skills) so I can't know in which direction to spec, or my equipment won't allow me to mix-and-match the gems like I want, nor link the support gems with the stuff I use, maybe I won't be able to level up my gems because of the requirements that sometimes won't follow how I'm making my character or the upgrades just won't matter. Corpse explosion, for instance, deals 20% of the health the monster had. +2-4 at first level, +2-5 at second level, etc. Auras such as Anger add 15% to your cold damage but slightly increase in aura radius as the skill gains in power. So you can't level them up sometimes and when you manage to, it's kinda disappointing.
Support gems aren't always great either, finding something nice you can't use because of item slots, because of level requirements or because of not having any attacks that can use it. Finding a 'Fork' gem is useless if you're fighting melee or using some spells that don't shoot projectiles, being unable to apply support gems to your basic attack is kinda annoying too because having a gem adding "Monsters hits have a chance to flee" on only one of your attacks that you might not use because of mana issues is disappointing (Also, gems add mana costs)
How I would fix this
Give more gems as quest rewards, give more options to mix and match support gems with what you're using, and finally make the gems have bigger impacts.
Unique items are uniquely designed, to be sure, some of them have downsizes, it's too bad that it's all I've gotten so far. I suppose there are tons of better stuff so I'm not taking it against the game.
All and all, it lacks charm
I'm not sure I want to continue playing path of exile and it's because of the first reason more than the second one, but if I had to find another problem with the game is the relative style of everything, everything is brown or dark or very low-colors. I'm not too sure what it's trying to inspire or recreate but all and all, I find it depressing to look at and I always hope they're going to change the style by the next patch. Of course, if the game was amazing otherwise I wouldn't care much, but the fact that everything feels slow or unresponsive doesn't help my view of the game and the style surely doesn't help.
I know it's a question of taste, but brighter effects and clearer graphics would've helped the otherwise glue-like gameplay.