RSS Aggregators are an interesting pain point for me. I'm never quite satisfied with my choices. I've used several over the years with probably my longest period spent using Thunderbird as combination email and RSS reader. The problem with an offline reader is it is hard to sync across computers. Thunderbird did however offer great keyboard shortcuts and I liked being able to quickly switch from reading new mail messages to new RSS items.
I didn't like Google Reader the first few times I looked at it, but the recent updates have been compelling enough for me to try using it regularly, and I was certainly greatful for the OPML magic offered by Thunderbird and Google Reader. I like the use of Infinite Scroll here.
Infinite Scroll was pioneered by Microsoft's Live Search, but since removed from traditional search. It was a neat technique, but caused more problems with traditional search than it enhanced. It still exists as a part of the Live Search Images searches. Infinite Scroll in terms of RSS reading allows me to simply scroll through all of the unread items, with Google Reader highlighting items as they scroll by and then automatically marking them as read as the scroll off the top. Combine that with ample keyboard shortcut goodness and I'm not missing Thunderbird all that much.
There are still a few things I find lacking though: search (funny for a Google product to lack a search bar, but Reader certainly does... I've had a couple instances already of trying to / search just like I would in GMail...) and this has been an annoyance to me since I started working with GMail even: no ability to sort New items oldest to newest. In Gmail that problem is only a minor annoyance because with email I will often do a quick scan for new important emails and the keyboard shortcuts and conversation views make things moot. For Google Reader, with its infinite scroll river of news items it is a much, much larger concern because if I scroll to the bottom and read up it marks everything as read before I get a chance to actually read it. Not to mention that scrolling all the way to the "bottom" is an effort and a half in the first place.