As with the launch of any new game there a few things you should expect to happen on the first day. Don't be alarmed, just be patient as everything will be worked out soon enough.
1. A massive influx of new (and returning) players to community. With any launch the first few weeks are going to be packed and the general skill level of players will plummet due to the arrival of the masses. Please be aware that new players are the life blood of any game. Don't be a dick to the new guys, keep them around. Answer their questions and be polite. Use this as an opportunity to get some new blood in to your regiments (if you need/want them) and welcome them with open arms in to what is hopefully a long lasting distraction in to our probably very hectic and busy lives.
2. A lack of general server space. Because of the influx of new players it may become hard to find a spot in a server to play in. If you do manage it, expect some latency issues as it will be a very heavy load on the servers. Be patient, and enjoy the next installment of mount and blade.
3. A drop in the general skill level of the population. Newbs are noobs, don't hate them, love them. Noobs are often the most dangerous foe you can fight because of their unpredictability. They are also most often really in to the game they just bought and are willing to listen to every piece of advice you give. Remember that some of these players will become long lasting members of the community, a good first impression on them could be the start of a long lasting friendship.
4. Teamkilling. It's going to happen a lot more in the next couple weeks than has happened in the last year. Because of the new faction, new uniforms, new classes, and new mechanics that the majority of us are unused to teamkills will happen. Prepare to be blown up by friendly sappers, killed by new players who don't know the difference between uniforms, and mutilated by vertical thrusts for the next several days. Stay calm, these are (hopefully) accidents and will become less and less common as the DLC ages.
5. Trolling. People troll. Especially in games that have just been released. Take chat with a grain of salt and don't be sucked in to idiots attempting to get a laugh out of players in a game they'll probably only play once or twice and then get banned in. The retards will be weeded out systematically by admins who will most assuredly be very active over the next couple months. Trolling unfortunately will never be completely eradicated, but soon enough our beloved home servers will be back to their good ol' normal self.
6. Complaining. This will be the scourge of the dead chat. While people are dead they are going to complain about everything changed or new that they are unaccustomed to until they get used to it. The vertical thrust, sappers, music, accuracy of weapons, reloading on the move. People will complain about everything until their complainers are all dried up. Don't get sucked in to their rants. Sit back and enjoy the game yourself and stay cool. Nothing ruins a fun launch like getting irritated by someone else who won't shut up, so don't forget there is an ignore function in game that will mute their chat so you don't see it.
I'm sure there are more and I'm just tired and not thinking. Feel free to add to my thoughts. See you guys in NW.