/sigh
I think the spell casting sea shells refers to the Spined Aloe's... the monk skilled creatures. I originally read the title of this thread and assumed it was in regards to the lack of difficulty through much of this game.
I will try to address your points in the format you provided...
Act I
Glad you enjoyed, I felt the same sense of wonder and success as I slowly progressed through the Ascalon storyline.
Act II
The Charr threat was not overcome; you, Rurik, and company left Ascalon because the King banished Rurik. Many of the folks from Ascalon tagged along with Rurik and fled what they thought was a doomed Ascalon. They moved with Rurik, and you, across the Northern Shiverpeak Mountains to Kryta. The storyline continued here with the introduction of the Deldrimor Dwarves, a group that you become very... familiar with later in the game in several missions, quests, and explorable areas.
Out of the critters in Kryta, only the Spined Aloes fit the description of sea-shells. These critters are simply a monk based monster, they just have different skill setups than the Charr monks from Ascalon. The mixture of spells you are describing sounds more like a bad pull of Skales, Aloes, and Mergoyles. Luckily, there are no hydras there.
As for the Fire Imps, Troll Ungent or Healing Breeze (if you have monk 2ndary) can reduce or completely remvoe the health degeneration caused by the burning attacks. The Fire Imps never presented a huge challenge to me but the Mergoyles did (I had problems with many Mesmer monsters). As for your powerful and carefully selected skill lists... no idea there, sounds like you may want to reconsider them if you are unable to use the henchies as a team in this area.
Not quite sure what happened to Act IV, is that gonna be in a sequel or something?
Act V
Rangers form an important part of the game, I have found myself kicking myself in the hind quarters for want of a Ranger. Rangers have a utility belt sort of skill set up, with good ranged, traps, self healing, condition inflicting, and even melee capability (through their pets).
As for the monsters getting harder... ummmm... look at nearly every game, cRPG, MMORPG, 1st persons, so on and so forth. Normally, the skill level and power level of the monsters does not decrease as the game progresses. You don't continue to advance and all the monsters just keep on being the same. I will admit that its not realistic to see a Level 10 collector standing out in the middle of a volcanic wasteland with Level 24 monsters milling around next to him, but it is a game where magic, dwarves, minotaurs, plants, and dragons are the like. The loot of the game gets progressively better, kill a Charr and get 8 gp than it becomes kill a mergoyle and get 30 gp. Though you have more slots for party members, you still don't have to fill them, and even if you did, 8/4 = 2 gp each... 30/6 = 5 gp each... the loot is progressively better. The xp for these monsters also goes up. I found that most of the experience in the game, initially, came from quests... kill 40 Charr with henchies get like 25 xp each, so 800 xp. Or do 1 Mission and get 1,000 plus a skill point, plus all the xp along the way, +1,000 more if you do the bonus.
As for going outside Kryta (I assume you mean Gates of Kryta or Lion's Arch) and leveling up 2 times... I can recall going outside Augury Rock and leveling up 4 times in a setting. Level 20 character, Level 22 monster... 136 xp each.
As for the enemies being there to annoy you... I suppose thats part of the idea... I'd hate to play a game where I just went from location to location without over coming anything, being handed xp and loot. The only importance of enemies is for small xp (until end game), loot, and to provide some modicum of challenge.
Henchman take a portion (equal share) of the cash dropped, they are after all members of your party. They additionally get a chance for any items dropped. As for the merchants offering PENNIES for super expensive stuff... ummm... I can't think of anything besides dye drops and a couple of runes I got in Kryta that were worth anything to anybody besides the merchant. Remember to hit the vendors with materials, dyes, and runes. And as for major runes, well, unluckily they are kinda like the red-headed step child of the game. Minors get a +1 bonus for no health, stick them until you can grab a single superior that you want...
As for the comparison to Diablo. I assume you mean D2. Diablo was a inventory and click game. It relied heavily on the mathematics of stats, far more so than GW. In D2, you simply loaded up on the currently available best gear went out and clicked until every opponent was dead. GW is a bit more about skill and using the right ones for the right situations, especially on the PvP end of things. Grab the best rune of vigor you can for one, stick to the minor runes to avoid the health penalties... The caster classes seem to gain a very largely apparent advantage from runes compared to the more subtle physical damage dealers which still get some great advantages.
Now in conclusion, not sure what to tell you... if you are not enjoying the game that much, I'd tell you to move on. Unluckily, most of the things you call problems are something that have been part of the game for a long time and have little chance of being changed. As for the noobish looks of the monster Aloes... I can't come up with anything to tell you. I hate to say something like this, but either buck up and figure out the game isn't about soloing every monster so you can get everything, its about finding a party, guild, and group of folks to go play and have fun with. The 20 levels, small skill bars, and so on make this game far less DIFFICULT than other games out there and make the game more about skill than endless grind.
Good luck.