For the love of Hydaelyn be mindful of spoilers, especially in FC/LS chat. Don’t be the jerk who ends up spoiling some awesome twist or emotional moment or amazing boss fight. To which end...
People are going to want to run things blind. If someone asks to, and you’re already at the point of revisiting content, please respect that. Let people enjoy fresh content, let them fail or succeed on their own surprises. There are not many chances in XIV people have to do exactly that. Let them. (EDIT: This doesn't mean don't be willing to help them and explain mechanics if you know them. But seriously, let people have a shot at things before you do so, at least for a while. I know you want things cleared efficiently; you have the rest of the game's lifespan to do so. Seeing things fresh is something you can only do once.)
The servers are going to suck. It’s common with every major MMORPG, both at launch and at expansions; and if an MMORPG doesn't have launch day issues, it's frankly not a good sign for the game's future. Generally, more people are going to be playing immediately following a release/expansion than at any other time in an MMORPG’s lifespan. Prepare yourself for that reality. You will probably get more lag than usual due to massive network activity. You will probably get login queues. You will probably get 90k’d (or people you’re doing dungeons with will). Brace yourself now.
There will be bugs. All the beta testing a dedicated QA team can do is nothing compared to the amount of playtime the game will get when it goes live. Be ready for the possibility of some really strange things happening, both good and bad.
Going to make a new alt/join another server? Create the character now, so you don't have to fight server restrictions. Once EA starts, the traffic on servers is going to pick up massively. With that comes increased character creation restrictions on servers - which, let's face it, will inevitably be on whatever server you're trying to get into. Create the character now to beat the rush. You can Fantasia over into an actual Au Ra once EA starts (you do get one Fantasia free from Veteran rewards) and not have to fight the system that way.
There will probably be an Emergency Maintenance at least once in the first couple weeks. This is simply inevitable, for hotfixes, server maintenance, etc. If this happens during the time you’re able to play, sorry. Remember that when they have to do it, it’s going to suck for someone - a different time just means a different group that gets angry.
You don’t have to do it all immediately. The developpers have already confirmed that, while Soldiery will be phased out, the new Tomestones for endgame content will be staggered out over the first few weeks after launch. This also includes the new endgame raid, Alexander (and even later than that, Alexander (Savage)). You don’t need to be running the final dungeon today. You can take your time. Remember this when servers are unstable or maintenance is going on.
People are going to suck at the game again. Sure, many players out there are fantastic. But let's be real - after months of being able to massively outgear the content we're tackling, a lot of people have gotten used to letting their iLvl carry them. That discrepency is going to fall off, and there are people who are going to have a hard time breaking bad habits they've gotten away with. Others will also be trying to get used to new techniques/classes and make mistakes because of it. Have patience.
The things you worked so hard to get aren’t going to be worth as much anymore. That max melded Forager’s gear? That crafting Lucis? That Zeta weapon? Those Dreadwyrm pieces? Be ready for the fact it’s not going to be the best anymore. Expect the costs and efforts involved in even making those from now on to sink as well. You should be somewhat used to this from patches as is, but the gap’s going to be even more notable. Get over this now, and remember you still have these cool things, and that you did it.
Rested Experience is a thing. It's that bonus EXP you accumulate when you log out in Sanctuaries. While the exact return/amount of time is something I've had trouble finding, if you want to ease up the initial levelling in Heavensward, you may wish to stop levelling low-levelled classes for a couple days prior to release. Regardless of if you're heading straight from 50 to 60, or going to start right away on one of the new classes, it's a leg up.
Just some thoughts. If you've other reminders you'd like to offer people, feel free to chime in. Let's not let our excitement be ruined by stupid mistakes or unrealistic expectations.