Previously Installed Addons

(The info here is current for version 1.0.0, which will be getting a full release imminently. If you're using an earlier version, definitely delete previously installed addons.)

If you already have installed addons that use plugins to load with the game (e.g. ArcDPS, GW2Radial, d912pxy), I would recommend deleting them. This is because the Addon Manager uses the addon loader to load plugins when launching the game, and the plugins get stored in separate folders, not in /bin(64)/. Because of this, any other plugins in /bin(64)/ won't work --- in theory, they shouldn't break anything else, but I'd remove them just in case.