It depends on how you make the game. Switching between classes is going to be easy. However, they have said that they want you to collect classes, so that might be the difficult part. They also want you to level up those classes to get more advanced skills (something that GW2 really needed to do). You also need gear to make those classes shine.
So in the end, what you may end up with, is everyone has the cleric class or whatever, but without the combination of class X, Y with gear item A, B they could only heal noob dungeons anyway so who cares?
Honestly, it sounds like they want Pokemon, but except instead of pets, you have class suits. Going up in tiers allows you to "evolve" your pokemon with unique abilities based on new armors and more efficient abilities. So like for example....
My
Rogue Pokemon: Sprint, Back Stab, Stealth, Gut Shank. It's tiered up to level 3 (Evolved)--which means I have Tier 3 equipment and my equipment has been crafted for (It sounds like equipment is high mod-ability in crafting) greatly increasing Stealth and Sprint. (Another person's Rogue Pokemo...I mean, "class", might have equipment that favors Back Stab or is more generalized.) And also all my abilities are "tier 3", which means my Back Stab is more powerful ect (Like leveling your skills in League ect)
However, lets say I meet someone with Rogue Destroyer Pokem--I mean class, that counters my build.
So now
Ice Magi Roguepokemon,
I choose you! I mean...I swap classes!
Ice Magi RogueClass=Back Stab, Teleport, Ice Bolt, Stealth! It's still a Rogue kit, but "evolved"...I mean, "geared/tiered" up with augmented gear to account for mage abilities. This pokemon is also tier 3, and counters the normal Rogue Destroyer Pokemon.
This sounds like how they are making the game. Classes are more like League/Pokemon, where they are picked for their strengths in a specific encounter. And like League, you can alter a classes function with gear--so it kind of adds a heavy RPG element to your little pokemon system. But essentially, PvP and PvE is going to be more about the breadth of tools you have, to counter specific mobs, rather than the depth of your character (IE more +Stats). So someone with 20 tier 4 Pokemon is going to a lot more powerful "in general" than someone with only 5 Tier 4, and 10 Tier 3 (ect) but in any
specificor discreet encounter, they can be even, because specific fights only allow one type of Pokemon to be used anyway. So the whole point of the game is to collect all your Pokemon (Classes) and evolve them into Tier 4--which due to the modability of armor, means every evolved Pokemon will be a little unique.
The problem with all that, is Pokemon and League don't require an identity in a world. Your pets are separate from "you". In this game they are combining you with those pets or champions--and I'm not sure if that will work well, because of how players respond to identity in games like this. But we will see.