Reeder's name alludes to it's purpose; it is an app for the convenient consumption of your Google Reader RSS feed. It has a few neat features, the most crucial being it's clean and highly readable presentation of text. While it may seem to be something you should take for granted it is the most important feature and the font and background choices are crucial. I actually find the pale, textured background of Reeder's iPad interface just slighty easier to read than the plain white of Instapaper (another favourite app). That's not all that Reeder has to offer though: it's real strength lies in integration. Reeder supports content aggregation and social/sharing platforms that I'm barely even aware of, the list is impressive. If you use services such as Reddit or Instapaper you are two taps away, the same with twitter or Facebook and numerous others. I tend to use Reeder in a very specific way: screening my feed for items that look interesting and then either reading them immediately (dependant on time and length) or forwarding to Instapaper for reading later.
It's only lack would be features for actively managing your subscriptions, but that would likely just serve to complicate what is currently a very elegant interface.