its very alarming but yesterday i was visiting some blogs (blogs w/less than 100,000 alexa rank) and 7/10 of them have their wordpress subfolders browsable w/c means anyone can view the url 'blogname.com/wp-content/plugins' or 'blogname.com/wp-content/themes' This is bad because hackers can pretty much exploit it and then hack your blog, or people could easily get your themes and stuffs inside your folders. The simplest solution is to put "options - indexes" in your .htaccess file...
Good idea! There are also plugins for additional security on your WordPress installations. One I use is Secure Files (http://wordpress.org/extend/plugins/secure-files/)
There are a few more, but that one is especially good imo. What exactly would you put in your htaccess to protect your files?