Everyone knows by now that Blizzard has split the entry area into two different areas (with four total Northrend entry points, one for each faction), and of course there's also Death Knights to roll, so hopefully the lag problem is helped (hard to believe it could actually be solved). But Relmstein has other ideas in mind: the dynamic spawn system (mobs will spawn faster the more people there are around killing them) causes some crazy repops last time, and hopefully that's been evened out a bit. He's also worried about the "leveling truce" on PvP servers -- during the last expansion, everyone was more interested in exploring than fighting for the first few levels, but if you missed that ad-hoc "truce," you had new level 70s beating you down as you were trying to explore the world. Hopefully the Lake Wintergrasp PvP zone will keep PvPers busy without griefing all of the people trying to see Northrend for the first time.
And later on, the Karazhan bump is a worry as well -- lots of guilds, early on in BC, were crushed by Karazhan's 10-man limit and the gear checks in there. Will 10-man Naxx also cause a ton of guild breakups, or will the 10/25 man split help guilds play what they want to play? We'll have to see how it all works out -- this is only the second time Blizzard has released this amount of content into the game, so while they're sure to have evened out some problems, you have to think that there will still be a few bumps in the road.