Well, since my main is a Ranger, I'll start.
I know I like to be able to see target HP, my HP, and group HP all fairly close, and my mana and stamina right there as well. I like to see my stats without having to open up the Inventory, especially ATK and AC.
I don't need a Pet Window (though obviously Shadowknights, Beastlords, and Bards DO), but I do like to have an easy to access spellbar, and hotbutton setup with all my inventory slots included. I also find it helpful to have all the weapon slots visible in the containers.
What about the rest of you?