Well... this whole topic is at the forefront of my mind because I spent quite a bit of time playing Path of Exile last year and that was also a game which prided itself on build variety without heavy class barriers (and, interestingly, has the same acronym).
But I also found this approach to be a major flaw. It was as if the game was half-minded as to whether it wanted to be class-strict or class-universal. You could be a Two-handed melee witch or a spell-casting ranged marauder, as the game made no rules to prevent this outright, but it put a huge amount of irritations in your way to deter you, such as attribute requirements which links to starting position and socket colour bias on equipment.
If the game wanted complete build freedom then it should just have one class with limitless 'path' choices.
If the game wanted defined classes then there should have been outright restrictions to the use of some equipment and skills for certain classes.
Having it half-assed with irritated but unspoken freedom just meant that a lot of players ended up making bad skill and passive choices, effectively gimping their character before they'd even got half-way into the first difficulty level - addicted to skills which required large amounts of non-class attributes and off-colour socketing.
Then they introduced a 7th class, the Scion (a standard hot babe) and made her a non-class-specific character. When I last looked at the forums the most active thread was one entitled "whatever your build, I bet I can improve it by starting as a Scion", to which, yes, the guy really could.
I'm not against such things as dwarven wizards and elven barbarians etc, just as long as they ensure that class distinction is either completely relevant or not at all - anything half-assed is just going to make people play something else.