Spam
People using email have to put up with email spam.  Bloggers have to put  up with blog spam. People write programs that crawl the blogsphere and leave advertisements in the comments and trackback sections of the blog. It is highly annoying by both readers and bloggers. Most people who hit blogs are ad-savvy, so I wonder who the hell clicks on these ads? The trouble with spam in general is that it costs the sender nothing to start the process. Think that if it cost nothing for people to send us physical (snail) mail we would need bathtubs for mailboxes.
You will run into filters that have a graphic picture of numbers and letters and ask you to read them and type them in to prove you are a human. I'm legally blind, so they are exceedingly hard to read. I've read that the spammers have better and better systems to read those funky graphics. Pretty soon they will have to use one of those dot-paintings you see in the mall that is of a sailboat. I've never been able to see those things either. I bet a computer would have an easier time deciphering those than people.
There are user registration methods where they send you a "bounce" email and make sure you are the owner of the email account you used. They can block IPs, but that doesn't work. For every trick the defenders of information put forth, the spammers find a way to thwart it. I bet this will always be the case. The only way to stop spam is to take the money out of it.