Oh god, so many
I've been in a brilliant relationship for 9 years now, but before meeting him I learned to...
Meet quickly - no talking for weeks/months and getting attached
Keep that first meeting short, even if I liked them. Just half hour or an hour max for coffee or whatever. Then if they are awful I haven't wasted much time and if they are weird/clingy/stalkery in person I have an excuse to leave
I think the first date when online dating serves to put the potential rapport there, that is missing from meeting someone say at a pub or from something to do with uni/work/etc
Until you meet in person you have no idea if they have lied about height, or age, or weight
You have no idea if they smell of fish or eggs or BO or 20 litres of old spice
You have no idea how they act in public
Some people manage to hide their teeth, or lack thereof in pictures as well
A quick meeting will establish all those things and whether for both of you, its worth doing a real first date or not