When you click on a feed in aggRRSive is shows the contents in a pop-up box, whereas in RSSor the contents is seen within the same window allowing you to do more with it like bookmark it, linkbacks in del.icio.us, see comments (also shows author’s name, category, and permalink).
[ADDED 20/10/05: To splice a feed just select from your list of feeds, then a heading Heart-Cart appears on the right, this gives you the option to make a merged feed, to do so select “Create an aggrssive” from the drop down menu.
These can be tagged as well, and they appear in your list, these spliced feeds are differentiated in your list as they have a heart icon.
There is also a bookmarklet to add feeds to your account.
Need an OPML to be able to share dynamic feed lists.
Every tag has an RSS feed, what if every tag also had an OPML, then you could add heaps of separate feeds into your RSS reader in one go.]