The thing that annoys me most is that now GMail by default shows images which can allow spammers and marketers to detect open rates and validate active email addresses.

This is made worse by the fact that marking messages as spam requires you to open the email first. By the time you open it, the spammer knows your email address is valid and active and even if you mark it as spam, your email will probably have made it to some other list of 'validated gmail users'.