The common mistakes

[HEC]

I think what you should have on TOP of the list is:

Communication!

Don't need to discuss strategies for minutes, simple "Ready?" / "ready" or "go" will do a lot in most of the situations. Even to say "Hi all" at the start of the dungeon or not to be ashamed to say "It's my first time as [put your class here] as I'm leveling the secondary class so please be patient with me" will do no harm. I tend to tell party "Please wait, running low on MP" although I know they can (and should) watch the bars in the party list and so on. As long as you're nice and polite and not trying to be despotic or too controlling even when pointing some "misshaps" of others you'd hardly ever get really negative response to that and if you do at least you know who you're dealing with ...