If you send an email message with few hundred lines to a GMail user, he may not be able to read your full message in one go as GMail automatically cuts-off the bottom portion of a long message.
To read the complete email text, the user will have to click the "View Entire Message" link available at the bottom of the email that will open the full message in another browser window. Not sure how many users will take that extra step and they miss the important bits of your newsletter.
Wait, there's another limitation in GMail - if that GMail user tries to forward your long email to another user on GMail, Hotmail or Yahoo, GMail will just forward the truncated text, not the entire email. And Google will not give any warning message before truncating that information from your email message.
The workaround for the GMail message limit ? Either use another email program like Windows Live or Yahoo! when subscribing to email newsletters or download GMail to Outlook / Thunderbird - the POP access will always download the full message body from GMail servers.