Controller
Controllers focus on affecting multiple targets at once, either damaging or debuffing them, or altering the battlefield's terrain. Some classes, such as Wizards and Invokers, are focused towards ranged combat, while Druids can specialize in ranged or melee combat.
Defender
Defenders focus on blocking attacking enemies and focusing their attacks on themselves. Defender classes are typically focused on melee combat, however some classes such as Swordmages also have ranged combat capabilities.
Leader
Leaders are focused on buffing and healing allies. Some Leader classes and builds are focused towards either melee or ranged combat, however the role as a whole is not.
Striker
Strikers are focused on mobility, dealing heavy damage to single targets and avoiding attacks. Some Striker classes and builds are focused towards either melee or ranged combat, however the role as a whole is not.
Comments
I think most people see that those four class categories you listed are just synonyms for the roles classes have in standard MMOs.
Yes people focus a lot on maximizing DPS, and why shouldn't they? It's typically the go-to secondary role for most classes because it's something everyone is able to do. Just because it's "Not your job" doesn't mean you shouldn't help/try if you are able to. Simply refusing to do something because it's outside of your primary responsibility is just lazy.
Yes, some people take this to the extreme and unrealistically expect everyone to play beyond their characters ability and those people are stupid. An example of this would be someone that expects a character that just got max level and has bad gear to put up the same numbers(damage/healing/damage reduction) as someone who has been max level for a long time, has the best gear and has extensive knowledge of all the encounter content which allows them to further maximize their numbers.
Also, yes, there are plenty of people that just look at the numbers and ignore the context. If your group is doing aoe fights and there's a striker/dps that can only effectively do single target damage then of course their overall damage output will be lower than others.
In these type of situations I agree, mindless trolls fapping over their dps numbers are D-bags.
Realistic people will never expect a defender/tank to put up the same dps numbers as a striker/dps unless the game allows it (ie if the defender/tank class is overpowered or the game is designed to allow for high dps defenders/tanks). But this does not forgive someone for simply not trying.
Also, for the record, I am not a hardcore raider... hell I'm not even a casual raider. So I'm not "One of those people", but I do aspire to do my best and do more than just my primary class role. Sometimes this is possible, other times it's not, but I won't stop trying.
*note* If you are specifically talking about D&D and not about Neverwinter, the MMO, then you can ignore my verbal vomit. I am completely ignorant to D&D 4e rules and classes and I do not want to impose an opinion that has no basis. But if you are talking about Neverwinter, the MMO, then I stand by my opinion.