Okay guys, your WoW is showing. You're talking about WvW as if it's structured PVP. This is a massive area, with hundreds of people fighting all at the same time. And it's a PVe area, in which combat is allowed. There's crafting nodes to gather, you can do crafting. In fact, you can level from 1-80 in WvW. So what's the difference is someone comes in and does a jumping puzzle. How is this different from people on a PVP server doing PVe at any given time in WoW?
There are dynamic events and PVe things to do in WvW because it's a PVe part of the game where PVP happens as well.
And how can someone grief that? The entire thing is set up to be casual and basically imbalanced. Servers that have less people on them get buffed to compensate, there's three factions, so the two weaker factions can go up against the dominant faction and take them down.
I just don't really see any way to grief WvW...at least no obvious way. Personally I plan to do WvW as well, but with a smaller group, taking mines and limiting supply to the keeps who need that to repair. One guy or even a dozen doing a jumping puzzle won't affect your experience at all.
Edit: Ah I listened further and see what you mean by griefing. I'm not sure that in WvW anyone will care enough to do that, but if they do, that's PVP, no? Because everyone is level 80, ganking is going to be a lot harder. And those that want to get there first, have to take the time to do the puzzle too. I don't see it happening very often.