I attend Fragapalooza on a yearly basis and they manage ~200 folks, I've volunteered a few times myself for setup / teardown and over the years some things have become apparent:
Having stable power distribution is your top priority, no matter how much you've solved other problems when power goes down it's going to kill everything. Worse yet if you have rolling power issues that's going to put a real kink in your tournament scheduling. The main thing to consider when it comes to power distribution is what kind of hardware is going to show up, if you are using tournament machines where every build is identical then it shouldn't be a problem, if people are bringing their own machines you're going to have to sort out wildly fluctuating power configurations.
Your LAN setup needs to be flawless, monitored and set up to find and eliminate problems. That one person who shows up with DHCP turned on is going to be a cancer, the faster you can find problems like that and solve them the better. You'll also need people to keep an eye out for hacking, tournament play, it happens
Problem 1: You're hosting a LAN style event with a required WAN connection, you can do everything in your power to ensure that you've got the bandwidth to handle X number of simultaneous players as well as whatever the players who aren't in the tournament are playing, even if you handle this perfectly online-only games are a bitch to run tournaments for because if the servers you are connecting to go down your event is over or will drag on way too long. Even checking for potential maintenance windows to ensure there's not going to be downtime during your tournament hours is something important that's easily overlooked.
Other stuff you're going to need to consider is gate security and floor security, not just for things like theft but also for ... conflagrations between players. When people get mad you need to be able to deal with them quickly otherwise things start to escalate, it's bad for your event, it's bad for your attendees.
Anyway, all this stuff probably seems obvious but it's hard to achieve AND maintain