As a Hunter, lag is inconvenient. I might need to wait an extra second before an arrow fires, and I will occasionally mess up my rotation because a lag spike lasted longer than an activation/animation time. Meh, I sometimes shoot people in the face a slightly different way than expected.
As a Warden, lag is fairly horrible. Gambits key off combinations of three skills, so you could be in trouble if thrown-off timing leads to your getting the wrong skills, the wrong order, or a gambit firing before it is complete. If I need my area effect morale leech (312-4), I may really need that health, and 31-4, 32-4, 12-4, or 312-“why isn’t this firing” are not going to cut it. Those are all other gambits, one of which has a small HoT, but they are not the one I need.
Server-side lag has been problematic since the last update. Every fight or two, everything freezes for 0.5 to 2 seconds. It feels absurd to whine about 1 second delays, but that is very frequent, relatively large, and annoyance builds up. Gameplay has a rhythm, and you do not want skips several times a song. I find myself paying more attention to the UI so that lag befouls fewer gambits.