Twitter's notification emails when you get a new follower have been in plain text since the site started. Today, Twitter finally rolled out a richer, HTML version of their follower notification emails. These emails include the number of people that this user is following and how many are following them back, along with their total number of updates tweeted.
This is a big step up from Twitter's old follower notification system. With the old system, you were just blindly clicking into their account with no knowledge of the user. While it's still not perfect, at least getting a look at the follower/following numbers can help to weed out the spammers. Ideally, Twitter should include the user's bio, along with their last few tweets so that users can really make an educated decision as to whether or not it's worth checking out the profile.
At this time, it doesn't appear that users can choose between plain text and HTML emails. Twitter seems to moved completely to HTML-based emails. While this may have been a concern in the past, realistically any modern email client or service can handle emails in HTML.