Easily counterable but strong skills
A lot of times bosses in Guild Wars will have really strong skills but their's no way we can counter it unless we A. Overwelm them or B. use A skill on them which only X profession has (this wouldn't be so bad if it was a mesmer that had the skill
). You should have powerful enchantments that you must remove or you get screwed, removes deadly stances or interupt long casting powerful skills (in hard mode they cast faster 
)Encourage reaction to audio and visual ques
Hear a big "FFFFOOWWWAAAAA" sound and see the great destoyer raise with glowing fiery hands. That means run out of Area range. Having a lot of long casting andmaybe uninteruptable skills that affect an area range area but are extremely easy to spot and avoid would be a wasy way to prevent players from falling asleep.
Warning beforehand of skills needed.
NPCs you meet should warn you beforehand what you can expect a boss has. Simple dialong like "Watch out for the Illisionist he'll reflect your attacks with his terrible enchantments" would give players a fair warning before they go through a mission only to find out that next time they need to bring enchantment removal.
Usage of hexes and conditions that need removal
When abbadon used "Word of Chaos" everything gets Dazed encouraging partywide removal. This is a good idea for making the monk's side of the battle more engaging. Simple hexs that double damage done to a target, kill if left alone, or just disarm a target would be nice. Also Conditions like Blind and Dazed can work too.
FEWER MINIONS IN BOSS BATTLES
It's REALLY REALLY annoying to have to interupt a boss battle to fight a bunch of creatures in the back. Sometimes this can be good like if the boss retreats while you kill a few of his minions and then he jumps back in or having them guard the one thing tha twould kill a boss is ok but having a bunch of them that you can have to avoid aggroing is very very very annoying.
Use the enviroment to harm a boss
What puts out fire? WATER! Theres a lake and a catapult at at yoru location accross a bridge but whoever goes has to fight a small group of monsters will everyone distracts the boss. It's a lot funner to use elements of an enviroment to damage an oppenent because it keeps you looking around for something to use.
Major Bosses don't regenerate health
I know this idea has already been taken into effect with Abbadon but I just wanna repeat it. Bosses who temporalilary back-away shouldn't regenerate health.
Encourage Teamwork
We are in a MMORPG! While taking down a boss there should be show coordination between teammember other then "EVERYONE ATTACK HIM". This could be simple as calling "I'm using Wild Blow on Boss" while someone disenchants him.
Example Boss:
Ok to show you how a few of the ideas I suggested would a make a good boss by making up one myself by creating a simple boss with a couple skills that should be fun to take down.
The Illisionist
The Illisionist is basically the ultimate mesmer. He doesn't use powerful damage spells to damage targets instead he reflects and manipulates his ememies to kill them. Attacking him slow is better then attacking him faster.
Legend:
C- Casting Time
R: recharge
E: Energy
Damage Reflection
C: 1/4, R: 20 Sec, E: 5
Enchantment spell. For 30 seconds whenever you take damage the creature that damage that attacked you takes double that damage.
This is his main spell that he spams constantly and you'll have to get around. So basically attacking him with a 1 tank and 3 SF etc and your screwed. He always tries to maintain this so enchantment removal is a must. This wouldn't be so hard to do unless he didn't have:
Preservation Stance
C: N/A R: 10 E: 5
Stance. For 20 Seconds you can't be hexed, and conditions can't be applied to you and all enchantment removal spells fail against you. Going into this stance does not remove current hex/conditions.
Makes things a little harder doesn't it? This would require a stance removal and a little coordination between the mesmer/nerco and a warrior/assassin/dervish/paragon.
And now for some fun spells you have to avoid
Mark of Frenzy
C: 1 R: 30 E: 5
Hex Spell. If target foe was attacking then he is hexed with Mark of Frenzy. While hexed target foe takes triple damage.
Better not attack him while damage reflect is up
Focus Snap
C: 1/4 R: 45 E: 10
Hex Spell. Interupt target foe's action. If that action was a spell then he is hexed with Focus Snap. While hexed target foe casts spells 100% slower and takes 200 damage when interupted.
You read that right elementists. Frustration on steriods
Illisionary Riposte
C: N/A R: 30 E: 5
Skill. You can't use this skill unless you are below 50% health. The for 10 seconds next attack skill used against you is negated (not blocked) and your attacker takes 100 chaos damage and is blinded for 10 seconds.
Got used to Wild blowing then disenchanting the illisionist. Ok good now before you kill him you gotta deal with this too
. Pretty much the counter-tactic would be to wait till it expires or trigger it early.Neglect
C: 1/4 R: 30 E: 5
Hex spell. If target foe is below 50% then he is hexed with Neglect. After 10 seconds target foe takes 300 damage.
Basically a finisher move unless a monk has anything to say about it!
"Hit Me!"
C: N/A R: 30 E: 10
Shout. If your below 50%. The next time you take more then 130 damage that damage is negated and you deal 250 damage to the attacker.
Reversal of Damage on crack.
Anti-ressurect
C: 1 R: 30 E: 5
Spell. If any foes in the area are casting ressurect spells then they are interupted and takes 200 damage and this spell recharges instantly. If there are 5 dead foes in the area this damage is doubled
The trick here is to make him use this spell but make sure he doesn't interupt you.
All in all this basically a boss that makes you kill yourself.
