Stats qua stats (i.e. in and of themselves) are important but understanding how and in what way your stats interact with your skill tree is, I would argue, much more important.
Take a look at skills on your skill tree (passives, I believe they are called) and think about the kind of play style you want for your character.
For example, I love the dodge mechanic and being able to blink, dash, and jump around a lot and wanted to build a character that could take advantage of this play style. I also like swords and think they are over-represented in nearly every single dungeon crawler I've ever played, right up from Diablo 1, so I am banking on them having more uniques and legendary variants than, say, staffs.
So I am building a dual wielding swords character and stacking Ferocity and Agility, which will maximize my critical strike chance and attack speed, so I can get lots of damage in over a short amount of time (burst) before jumping out of the way of incoming damage (making the build that some RPG fans call a "glass cannon"). There is a major green passive node on the 2nd rung that makes critical strikes deal 100% more damage, but reduces your characters damage by 30%.
This would be a terrible passive skill to choose if your critical strike chance was only 5%, and is mathematically and demonstrably bad if your critical strike chance is under 30% (assuming that "normal" critical damage is 2x normal damage). As soon as you start getting that critical strike chance above 30% however, this skill starts to make sense. Why? Well, now for the math part...
Let's say you deal 100 damage per attack of your weapon (weapon damage), and your weapon swings once every second (attack speed), and that critical strikes normally deal 2x damage (critical strike damage).
With a 10% critical strike chance:
- at normal (unchanged) damage you will deal 100 damage per attack and 200 damage once every 10 attacks which totals 110DPS
- with the passive skill you will deal 70 damage per attack and 280 damage once every 10 attacks which totals 91DPS
With a 33.3% critical strike chance:
- at normal (unchanged) damage you will deal 100 damage per attack and 200 damage once every 3 attacks which totals 133.3DPS
- with the passive skill you will deal 70 damage per attack and 280 damage once every 3 attacks which totals 140DPS
With a 50% critical strike chance:
- at normal (unchanged) damage you will deal 100 damage per attack and 200 damage once every 2 attacks which totals 150DPS
- with the passive skill you will deal 70 damage per attack and 280 damage once every 2 attacks which totals 175DPS
Now although all stats may contribute to your damage output through things like ailments (bleeding, poison, etc), characters that have a super high wisdom and toughness would not really benefit from this particular passive skill, since neither of those stats increases core weapon damage, attack speed, critical strike chance, or critical strike damage (the four fundamental factors used in this equation).
By understanding that Ferocity and Agility directly boost those four fundamental factors, and that Wisdom and Toughness do not (by themselves), players can understand what choices to make when selecting passive skills and assigning attributes when they level up.
That said, don't be afraid to experiment with different build styles and active skills too! I am currently using a skill that requires a dagger (so am using one sword and one dagger) just to see how that feels with my character. If you really do feel like you are getting your ass kicked or it takes forever to down enemies, take your character to a safe place (like town) and spend half an hour or so figuring out how your attributes interact with your chosen passive skills.
Worst case scenario you could always pay the refund cost and start fresh on your already leveled character!
Edit: messed up my math the first time, corrected it.