Well some multimedia plugins will make the page not look right on some other browser such as FF or Opera. Such as flash anims, may be possible to see on yours, but older computers, nope. Another drawback is that consumers hate having to download all these plugins to view certain content. Also on some forums there is an automated message plugin...basically myspace or so, i know you can disable this but everytime you send/receive a message you will get a confirmation email..thats another drawback.
some thinks like .htaccess drawbacks also..hacking stuff.