It's possible to combine and mix your skills

[Emilylowes 0]

From an entertainment standpoint It's an entirely personal choice, but there's an explanation for  D2R Items why Barbarian was the only returning character from Diablo 3 at launch--Blizzard thought there was a lot of potential for improvement in that one. The lack of information available on the screen to create characters doesn't mention this, however there are a variety of class designs and completely diverse ways to play the character of a particular class.

The wide variety of options implies that each class will perform in a different way depending on the options you choose. For instance, with the Necromancer You could focus solely on summoning a gang of minions. Alternatively, you might choose an option that lets you invest points in poison/bone spells which allows you to deal with the damage more quickly. The Assassin can concentrate on the traps they create or the martial arts. Druids are shapeshifters and rely on elementsal attacks. The class you choose to play depends on it they can also be able to center your entire game around one skill like Zeal Paladins, or the Blessed Hammer Paladins. They're popularly referred to as Hammerdins.

While it's possible to combine and mix your skills however you like but it's always recommended to create a strategy and then invest according to the plan. There's a finite amount of points you can earn which means you won't have the ability to learn every skill in the tree. Even if you decide to invest a lot of time and money into a specific ability, it could dictate the other skills you select due to synergies between skills that are highlighted when hovering over a particular skill. For example, the fundamental Raise Skeleton ability for  cheap d2r items Necromancers receives a bonus due to Skeleton mastery as well as Summon Resist, which makes the skills essential that summoners should choose.