I've been soloing the new content primarily to get to see the quests in detail before I go out into the wilds of PUG groups these days, if for no other reason that I can take in the look and detail of things (as well as get familiar with the quest) before I get into a group that will more often than not rush to the finish line. But when content becomes familiar territory, then its par for the course to get through it. I paid good money for the expansion, so I will do it on my own time to really get to see the new stuff before it becomes the rote of "zone, haste and go" lol.. I am fully capable of keeping up to zerg runs, but I can't keep a breakneck pace like that forever.. its' burned me out before, so I am instead keeping it selective to when I go and when I 'woah'.
Getting more to the topic at hand, I feel that the key points would be "new to quest" or "need guide" would be flags that zerging is not in the cards.. better to lean on that the leader would want a slower paced run to get the feel for the quest instead of seeing it go by in a blur; doesn't mean that unless they explicitly state that zerging is not an option, that it gives you free reign to do it anyway.. just avoid that LFM, then post your own or wait. Saves a ton of headaches on all sides.