This is an interesting topic. I turned off trackbacks/pingbacks on most of my blogs (personal and CMS ones). I agree no one seems to use trackbacks/pingbacks properly anymore. Almost all the trackback/pingbacks I see are from untrusted sources that I end up moderating as SPAM. There are occasional real trackbacks though, so I don't necessarily recommend turning it off on all blogs, depends on what you're doing.

Thinking of DoS attacks, every page request for a WordPress blog requires PHP/MySQL to load, so depending on how much horsepower your server has it sometimes doesn't take much to make a server go down. If you're using shared server where you don't have access install real caching such as APC on your server, then you're only option is a caching plugin. Even if you use a caching plugin your blog still uses CPU/memory because PHP/MySQL still load. There are some caching plugins that do true caching with very complicated mod_rewrite rules in the .htaccess file, but using such caching means you cannot use a lot of the cool plugins that update dynamically (twitter and rating plugins come to mind).

If you have full control of your server, you're better off installing APC so you get true caching of PHP in memory. There are many other tweaks you can make to MySQL as well to facilitate caching of the most queried data. If you have the ability to do these things, there's absolutely no reason to use a caching plugin since these applications like APC do the caching at a much higher level.

For blog feeds, I wrote a plugin called Static Feed that lets you serve your primary feeds as static XML files. This gives you the advantage of being able to serve your feeds even if your PHP/MySQL fail due to a DoS attack. I use it to remove the extra CPU/Memory load from having WordPress dynamically re-create the XML feeds every time the feeds are requested. The Static Feed plugin works by saving an XML copy of your feed any time you edit, add or delete blog posts. If you're only writing 1-3 blog posts a day, you're only resulting in a handful of MySQL queries to your database and saving a lot of CPU/Memory usage from loading PHP. Best part is my Static Feed plugin can either be configured for permalink style feeds (e.g. example.com/feed/) or Moveable Type style (e.g. example.com/index.xml).