Many people forget (I myself have) that before dreads are made, there is some preparation involved.
WASHING. Before I had my dreadlocks, I washed and conditioned my hair almost every day. Without slowly weaning your head off of washing, your scalp will be horribly itchy. Dreadlocks in their early stage will probably be itchy, and (in my case anyway) may be itchy in the middle of the night. Start preparing 2 months in advance if you wash your hair on a daily basis. Skip a day or two, and continue to do so until you can go over a week and a half without washing. Do this, and you can prevent some very agonizing hours of itching.
MENTALLY: Are you ready to be "that guy/girl with dreads"? Just as you may identify someone by their tattoos, piercings or brightly dyed hair, people will identify you by your dreadlocks. You will probably notice during your premiere public outing that almost everyone will notice you. Suddenly your presence in any public place is noticed. After a while, you'll probably stop noticing how much attention you really get. It's not so bad if you really love your dreadlocks, and are confident that people are looking for positive reasons. Not to sounds snobby or overly confident, but if my dreads spoke, they would probably say something like "yea everyones looking at me. No big deal. They LOVE me." :) Generally speaking, I'd say people get a confidence boost after getting dreads.
FAMILY: Not so much an issue for me. My parents are very supportive of my dreadlocks (surprisingly!). Everyone is entitled to their own style, but family can become an issue. If your not ready to hear what your family may tell you, then maybe hold of of the dreads.
NEGATIVE COMMENTS: People generally won't say that they flat out don't like your hair, but its always obvious how they really feel. You will get negative comments, but you will also get many positive ones as well. Sometimes you just have to suck it up and accept that not everyone is going to like it, and that the important thing is that YOU love them.