A level of difficulty entails certain assumptions. Problems arise when those assumptions do not obtain. Most difficulty settings assume some level of experience, either in player skill or numerical balance.
Numerically, a new raid tier assumes a certain gearscore, probably that almost everyone on your team has a partial set from the last raid tier. You could shoot lower or have a DPS check that assumes full tier 12 before you have a reasonable chance at tier 13, but that will lead to easy/difficult raids. The new raid will also assume that you already know all your class abilities and how to use them to counter common situations. Hard mode for the new raid will assume that you have beaten normal mode and know all the mechanics.
Pixel click bosses assume that you already know everything the bosses can do. Well, no, they don’t assume that, but beating them is balanced around that. You are expected to research or fail the first few times until you know what that icon means. If you have the time and resources to spend on first-hand research, you can be a trailblazer. If not, the wiki and Youtube are there for you. If difficulty were tuned to give you a reasonable chance walking in blind, you would probably find the fights trivial when you did not need to spend two minutes reading abilities and thinking about how they interact. (LotRO’s “In Their Absence” update did many things very well, including hitting this balance of fair bosses.)
When I say that Guild wars expects you to have the wiki open, I mean that the difficulty of encounters is tuned around players’ already knowing what those encounters are. You can beat many/most of them going in blind, and the mastery reward almost certainly involves knowing the encounters once you are past the tutorial missions. Later missions are balanced around the assumption that you have capped your equipment and that you have taken time to farm elite skills. With the right skill setup, missions can go from a 5% chance of success to a 95% chance of mastery. Even the hardest missions are not balanced around the assumption of the perfect build, so the perfect build can make it trivial, but an all purpose build may not work for all purposes, and it certainly might not get you to the mission bonus needed for the titles.
Most of us come from a single-player game background, and the “gotcha” moments there are so common that it is actually shocking to have a one-phase final boss. Oh look, I needed to bring two entirely different weapons to the final fight, and I lost half my health for not leaping away from the boss the instant he died. Yawn. Okay, learn how phase two works, then re-load and re-do phase one. Let’s hope we didn’t waste too much time on phase one. Do I sound bored? I’m bored with it. It’s both obvious and nigh impossible to plan for, so the game is effectively taxing you X minutes by not having a save point between boss phases. You know it’s going to try to screw you over, just like you knew a big fight was coming when you found the stack of health and ammo.
That moment is far worse in multi-player games because it needs to be balanced around everyone knowing, so being the one guy who does not know means ruining it for everyone. That’s not fun design. You face lots of situations where there is a briefing before the fight and half the players are just following orders while trying to get some sense of what is going on here. That’s not a lot of fun as either the leader or the follower, and there is a narrow window for groups that already know each other to explore collectively without anyone feeling dragged along or held back … but you’ve already heard my rants about games that need you to bring all your own friends and fun to work (short version: so you might as well play anything because the game is not carrying its weight).
The numeric balancing is clearer. You can demonstrate that X DPS or Y gearscore is needed to reasonably beat a fight. The developers could even post that on the entry screen. Skill and knowledge balancing is much harder.