Do you think all enemies should give XP?

In my opinion, allowing XP from every monster, instead of the system where you do not get XP for considerably weaker monsters, would open the door wide open for leveling bots. If you could get XP from monsters that can't even hurt you, then it would be extremely easy to run a bot for a few days and come back to a level 20 character. (Making the bot would be hard, but the usefullness of one would increase greatly)

The great thing about GW is that there is really no use for a bot. If you want great items, you can't really trust a bot to solo in those hard areas, and since XP fades off when you are a higher level, you cant level bot. Its really a smart system IMO

There are rewards your aiming for reguardless of where you go, if your swimming in a sea of weak enemies for nothing then it's your own fault, if there is a reward at the end of that quest your trying to complete, then you are getting something for it. Everyone knows that the vast majority of EXP is gotten in quests, not hacking, this promotes story completion in order to gain EXP, so your getting involved in the game instead of doing the grind.

Better system, but if you like grinding though, theres this great club on Peachtree St.

I definitelly agree on that. Besides, realism is messed up anyway. If devourers can already see who in your party is a main monk, and attack that person, then the realism in gone already. Then monsters should also be able to see your level and decide not to attack.
It works that way in quite a few other games and it works quite perfectly. There isn't really anything to cheat on that.
The only exception on the no-aggro rule might be missions. As they are meant as a fighting scenario. The no-aggro is meant mainly that you don't get swarmed when you need to walk somewhere unimportant by an immense swarm of pointless low critters which kill you but don't give a reward.

they should implement a system where if you complete a quest/mission with less players than recommended you gain extra EXP

Sorry didn't read the whole thing, but how about making the circle around your character much smaller around lower level creatures. That way you can avoid them easier. Also make it where the monster has to be in your circle longer to get aggro.

Also maybe ingnore then henchies too because they can't seem to walk in single file line and not want to fight.

Just something to think about.