it is a etiquette issue as a group was there getting ready to kill him and the other group NEEDED to wait its turn. i can understand why you dont know this as you are part of the problem having never learned this old world boss rule. it is exactly like i said a popular mmo never taught its player base etiquette and now games must be designed around it. your ffxi example is a prime example of the lack of etiquette. just because it was the norm there and in wow doesnt mean it is the "norm". this is how guilds get black listed and have problems recruiting. a majority of people dont want to play with a bunch of richards.
You missed a key bit.
I would bet that the other group did not even see the OP's group as they were not even close to gathered.
An organized group would likely pop from one to the other only taking time to distribute loot and check to see if the spawn is up and ready to go. The OP's group appears to have not even been in zone and they only saw some random dude staring at Dreadtooth. It would be different if the OP's group was fully formed and he had told the other group, "Yo, we had a guy d/c when he reconnects we are going to pull. If we wipe, you'll get a shot," or something similar. From what I gathered most of the raid was not even on the planet yet, and honestly I can't blame that all for taking the mob. I would have a lot more sympathy towards the OP if the big bad organized group ignored them when they were a man short or if they pulled shenanigans to cause a wipe, but from what the OP has said that was not the case. The other group was faster and better organized.
As far as popular MMOs go, I have been active enough to do world bosses on three games. WoW (through Burning Crusade), FFXI (through Chains of Promatia), and SWTOR. Of them people have been the coolest about world bosses in SWTOR by a country mile. The only times in WoW I had issues during World Boss fights were from Cross-Factional interference, which frankly is part of the thrill of doing them in a PvP setting. There is a competitive aspect to doing open world bosses, and if you can't handle losing the occasional pull to another group you might want to just stick to instanced content. And crying over Dreadtooth is especially childish as he has a very fast respawn timer as far as World Bosses go.
Lastly, etiquette does not necessarily mean "being nice," it is just the socially accepted rules of conduct. In the case of FFXI, the heavy competition and all the tricks involved were within bounds of etiquette, especially there as the Japanese players were on the server first, pretty much made the rules, and were very much into acting properly. I would go so far as to say that most MMOs could use a dose of Japanese-style MMO etiquette. It would put an end to the high stupidity that plagues Fleet Chat for hours at a time.