This is a great thread. I would like to chime in with a request to vendors:
I don't care if your labels are fancy or plain, coloured or monochrome. Other posters have made good suggestions of the information that should be on a label.
Please make them waterproof.
E-liquid is, as the name implies, liquid. Some of us with not-terribly-nimble fingers have been known to spill a drop or six. If the spillage of a minute amount makes your entire label illegible, you are doing your customers AND YOURSELVES a disservice. I have about a half-dozen 10ml bottles from one vendor with an excellent reputation on this forum. I had them all in a box and the labels got moistened with some residue or other. All of them are now pretty watercolours. I can't tell the hazelnut from the vanilla from the coffee. (Before you tell me to perform a sniff test, did that, but all these flavours are pretty subtle, so it's tough.)
It is in your best interest to have labels that can tolerate normal (which is to say, heavy) use by vapers. No good selling a product that the buyer can't repurchase because (s)he can't remember where it came from.
I will also echo lasttango's thoughts on "extra flavour" options. They scare me. If I don't order it, will I get a weakling water-flavoured juice? If I do get extra, will it be nauseatingly overpowering? What about those vendors that offer "SUPER" extra flavour? "DOUBLE SUPER"? "MEGA"? Etcetera. I usually pay for extra flavour if the option is available but be sure to include a note saying to only make the juice as strong as is appropriate without distorting the flavour. This has served me pretty well.
In short, don't send anything to a customer that you wouldn't vape yourself, Besides that I have been 95% thrilled with the service I have received from most E-juice vendors.