The vDip interactive map is pretty awesome. You click a unit, it will bring up some icons (for move, support hold, support move etc) and once you've clicked one, it highlights the possible provinces for that action. Other than the support move being unintuitive (you need to select the province you're supporting the move *into* before selecting the unit you're supporting) it's excellent and extremely well designed. I would very much like to see that over here.
The PlayDip order entry system, on the other hand, is awful. You click on the map and get a list of options (like vDip) but once you've selected that, the rest of the process is done with no feedback to the user outside of a small line of text at the bottom of the screen which you have to scroll off the map to see (convieniently, right next to the advertisements they put on every page), and once you've entered the orders there's no way to preview them on the map. It really makes you appreciate how good webDip's order entry system is :)
I'll echo the sentiment that, while the messaging system itself on playDip is not very good, the possibility of setting up a chatroom with multiple players is fantastic. I used to think that this would result in too many triple alliances, but in reality it's very easy to say one thing in a 3-way chat while turning the other two against one another in the 2-way chats. I'd love to see it added to webDip.
Variants... Fog of War would be awesome here, but that's obviously really difficult. PlayDip doesn't actually have Chaos - they have another variant that they named 'Chaos' which is actually the variant 1897 - but regardless, Chaos is quite fun. I've already proposed a variant rotation to the mods & Zultar, so I guess we'll see whether that gets accepted.
vDip's concede button would be a great addition for 1v1 games.