- At least choose a font for your title.
- "can probably be compiled on many other UNIX-like operating systems" comes across well from one nerd to another, but probably not to managers. This gets into the question of who your target audience is.
- In the intro, talk about what tarsnap does instead of just saying it's a secure online backup service and talking about the "client code". What makes you different from the hundreds of other backup services?
- The red yellow and blue color scheme isn't doing it for me. Looks better if you make the red yellow, but I know you want to draw attention to the call to action.
- The gap between the left column and the news is ugly. Consider not having news on this page at all.
Ok, that last one is facetious -- but in all seriousness, I'd love to hear something more specific than "unprofessional".