Path of Exile is a much better game

I switched back to Path of Exile from Diablo III after growing tired of the game never changing, the game had changed quite a bit but Path of Exile is a much better game now I'd say.

1.) This is down to you entirely, builds in Diablo III are very easy to modify whenever you wish but this is not the case in PoE. You spend points on the skill tree and whilst you do get some re-spec points that allow you to change things, making large changes to a build will require the use of a currency known as an 'Path of Exile Orb of Regret' to give you more re-spec points.

Without trying to overload you with info I would suggest you take a read through the active skill gems on the wiki, see what sounds interesting to you and make a build selection based around that. If more than one sounds interesting then pick an archetype (e.g. melee, caster, summoner etc) and fiddle with skills til you find out what you like.

Unlike Diablo III, you aren't restricted in skill selection based on items giving bonuses to ability X but not ability Y. So in Diablo there would be no real reason to use say the DH skill Multishot if you're playing the Natalya's Set because there are better sets for that skill. In Path of Exile you can make most things 'work' but it depends what you're trying to achieve.

2.) This is a bit of a sore point for some of the community but Path of Exile is a bit closer to vanilla Diablo III here, you go through the story 2.5/3 times and then get to endgame.

3.) This will depend on what general ARPG knowledge you already have but I'll highlight some important stuff: the economy is based around a variety of different poe orbs that all have their own value against other currency - think how the US$ will have a given value against the Euro but will have a different value against say Japanese Yen. Trading is largely unrestricted unlike Diablo III so if you get a good drop then you can safely store it away and know that you can give it to a friend or sell it later. Defensive mechanics are a bit more complex than Diablo III in that there are essentially two different health bars you could potentially have: Life and Energy Shield - I won't go into detail about them but generally you stack up on one or the other, life is easier to understand and closer to it's function in Diablo III so I'd start with that (so ignore builds that mention 'CI' or 'ES').