"The issues with invisible players in WvW comes down to a couple of factors. One is to do with when the servers notify any given client about other characters in the game (I'll call this reporting/culling) and the other deals with the time it takes the client to present the character on the screen after it has received that notification (I'll call this asset load).
The asset load issue is all about how quickly the client can show you something it knows it should show you. We're looking in to ways that we can make that process faster, but it's always going to take non-zero time. However, that's not where the bulk of the invisible players come from. For that we have to look at the reporting/culling side of things.
In the situation where the local population density is relatively low, when another character comes in reporting range of your character the server sends a notification to your client with all of the data that it will need to track and render that character. That includes appearance data, race, gender, profession, weapon sets, etc. etc. It's an easy model to think about and works well, until the local population density goes up. When there are a lot of characters in reporting range we start to get into a situation where, under the current system, there's an overwhelming amount of data to send to the client (hello n-squared problem!) - after all, resources like client CPU/Memory/etc. and bandwidth are finite. In order to deal with this situation we elected to change the criteria for reporting characters slightly. Rather than just using a fixed range we instead also limit reporting to the closest N characters. By doing this we help to ensure that we're not overwhelming the available bandwidth and, since clients clearly can't draw or process characters they don't know about, we get some savings on client performance "for free". This works out pretty well in PvE and doesn't seem, at least as far as I can tell, to have a detrimental impact on gameplay.
Unfortunately in WvW, where large battles are quite common, we find that players are bumping into these limits quite often and the effect has a real, and unfortunate, impact on the gameplay experience. Do I like the state that it's currently in? No, honestly I don't. WvW has been my baby (though not mine alone!) for quite a while now and I really want to see EPIC battles play out in all of their glory. If there was a switch I could flip to just make this work then I would have flipped the hell out of it by now, believe me. However, when you're dealing with resource utilization issues like this there are rarely any easy answers. I'm currently, actively looking into what exactly our options are in this regard. The fix is likely to be large-ish in scope, with changes on both the client and server, but we have some ideas that look like they may work out. So, will it get better? I can't make any promises because we're still experimenting and building new tech, but as a person who works on this every day I certainly believe that it can and will do everything in my power to make that happen."