For me, initially it was finding decent mailing software. I have a decent one now, but had to outlay for it – you get what you pay for really applies here.
For about 4 or so years, it was so frustrating to mail successfully, because either software problems, hosting problems because of the amount of mail going out.
Getting email addresses isnt the problem if you have the network of sites, and maintaining that list isnt too much of an issue as long as your software is good and you mail regularly (1-2 times a week).
Actually building up that initial database of emails can be a bit of a pain, but once its sizeable and returning – its a breeze!
Just needed to persist with it.