I rarely login to Feedburner even though I use it for my blog feeds. It’s a good service. I have a very specific critique though about the homepage. Every time I go to their homepage, I look for the button to login. I look all around the page, mostly in the blue area next to the flame. That area has a huge eye magnetism. (The eye is drawn to it). Additionally, the big Hello, our name is FeedBurner has a huge draw for the eye. I look and look. Finally, I realize that it is above the big bold area. The color and font of that area makes it look like a toolbar. Something meant for the chrome of the browser, not the webpage.
All they would have to do is put a dark black or red up there and I would find it much faster. The lesson is: If you want someone to find something, don’t “hide it in plain sight”. Make it stand out. I bet a simple color change would increase the number of new registered users significantly.