first of all, let me start out with saying i love this game, and im not ragequitting or asking for 3 of the four classes to be nerfed.
on the contrary, i would actually like others to read my suggestions, and bring up constructive critisesm. anyways trion, here is what i feel you could be doing better.

i like the idea that you are gradually pushing people towards going deep into one soul, and then choosing 2 supporting souls for that, it should make the game a lot easier to balance both for pve and pvp, so i would suggest adding a little carrot on a stick in some form as a reward for say, going at least 40 points in a tree.

your stat weights are screwed, you know it and you're working on it. nuff said.

the warlord soul for warriors are currently not really a viable tanking soul, and not really a viable support soul, so noone really goes deep warlord, i suggest reworking the tree from the 3'd step and up, making the warlord a dedicated SUPPORT tank. giving abilities such as soaking part of the main tanks damage for a few seconds, relocating threat from them selves, or a dps to a target of their choice, buffs that increase hp of party, calls to decrease aoe damage for the party for short bursts of time, and a rallying call removing fear or mind controll from a target, etc. as well as having the hp and armor to pick up adds, and drop them off on the "real" tanks by switching aggro.

clerics need to have their dps fixed, i dont have any really great suggestions as to how to do this without creating a godmode pvp class, but i imagine you could greatly buff abilities very deep into the trees.

you've got a point with the master modes, content recycling is a great way to fill the need for a change of scenery in between new content, do this with the t1 dungeons instead of the t2's, people would revisit them, if only for a while. very soon everyone will have hammerknell on farm (or well, all the "hardcores") so extend the master modes to old raids. ramp up the difficulty to hk levels, and give the gear from master mode raids a slight boost in stats. more content, and not to much itemization, or creation of new mechanics.

rifts are great, but they become stale quite quickly, you could add in more bosses to each plane, and then let rng decide which ones we end up fighting, some of the bosses from expert rifts would work just fine as raid rift bosses, if scaled to size, and vice versa. this would also play into the lore, telling players that they never know exactly what will happen when they open up a rift into the plane of death (or life or fire ore....)

in a lot of fights you give us a ton of visual clues to what is going on, but almost no audio clues, since it begins to get confusing when you have to watch text emotes, cooldowns, raid frames, cast bars, aoe, spell effects, text clues from the raid, flying damage numbers, all while rotating your view 360 degrees regularly to keep an eye on everything else, a piece of text easily gets missed, or the tiny castbar. on the "simon says" fight in GP for instance, a spooky voice telling you what to do would play right into the feel of the fight. where as the raid rift lady that casts downpour often kills all melee because half of them dont notice anything before the effect markers apear, here you could have the markers appear at the beginning of the cast instead of the end. making it easier to notice what is happening doesnt have to make for lower difficulty, but could instead be used as an instigator to more complex encounters.

you need threat indicators of some sort

user interface:
its better than in most mmo's, and you allow us to move and resize. but its not enough. it takes up too much space with fluff on small screens, and on big screens it looks clumsy in my opinion. and i think you are allready working on a fix to this through the addon api. take your time and do it right please

this is still a hassle, a big one. i could suggest a couple of solutions, but the two best ones imo would be to decrease aggro range on max lvl players from mobs below their level, or rework mob pathing in certain areas to stay clear of roads.

actually i have more, but this giant wall of text just critted me for over 9000, and knocked me out. thank you trion for making a great mmo, i hope you read and review my rant at some point in a not to distant future.