Coming from Dota, a huge part of my game was in the picking stage and basing how I was going to play the game based on what "heroes" everyone else picked (and therefore the overall synergy between myself and the 9 other players in lobby). I notice in guides from plat gw2 players that they always look at the icons at start and try to determine what roles the other 9 have, and therefore how they need to play. This is especially important as a thief main I would think.
So I've been studying a bit, and I've got the basic concept of the 4 referred to roles- DPS/Sup/Duel/Roam.. I get that this pigeon-holing is a little bit simplistic with some builds being hard into a particular role, and others having more license to improvise depending on the situation, but at this point I just want to get a handle on this first part. In order to do this, I need to first figure out what the general meta notion for class -> role is so that I can put that downtime to good use like I used to do in Dota.
The generalities I've picked up are things like:
-thief is almost certainly roamer
-necro is almost certainly dps
-guardian builds are usually some form of teamfight (either dps or sup, or blend of both)
The others I have some vague notions but far from certain. I haven't finished my qualifiers yet for ranked but I suspect I am in high silver / low gold meta based on guildies I've played with and their ratings.