The timing of TESO's launch is a point in time where people who play GW2 have not yet exhaust it's contents, while at the same time, already invested significant time in it. So the key question is, can TESO overcome these disadvantages to steal players from GW2?
Both of them have PVP as the endgame content. The average person who plays GW2 will just be about PVP ready when TESO launches. So TESO PVP has to be miles better than GW2 PVP in order to compete. What we know is that Arenanet has a lot of experienced in PVP, tons and tons of experience from GW, tons and tons of GW2 PVP testing by formal top GW PVP players. We have no solid information on Zenimax Online's experience in balancing PVP, but it is not unfair to say it would be extremely unlikely for TESO to have a better balanced PVP than GW2.
At the visuals department, from what we have seem from the leaked TESO in game graphics, GW2 is directly superior. GW2 visuals serious looks far more like a TES game than TESO itself.
Finally, the subscription mode of GW2 is a huge draw, in that it has NONE. We do not have details of TESO's subscription model yet, but the very best they could do is simply equalize with GW2.