I rarely noticed top guilds being "toxic" they can be sort of cruel. I've seen someone raid for six years with a group being sat for a trial who just did more damage. There are rules that I imagine a lot of less progressed guilds wouldn't tolerate but it was all handled rather professionally most of the time. There were a few that reacted badly but most understand it isn't personal at that level.
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I'm torn on this. My experience with clearing mythic or before mythic heroic was from tbc to wod with very few exceptions. I have noticed a ramping of difficulty but I'm not sure how large a jump it truely is. I can't say I found the last bosses of mythic znoth ( cant recall the raids name walking city?) To be on par with mythic HFC.