Well, aside from the look, which I don't particularly like very much mainly because I hate non-native looking programs in general...
I agree it would be a good idea to merge the shields like RejZoR suggested, it would simplify the GUI a bit. Of course, most of the settings should still be there, just put together.
Now, the main annoyance from the very beginning is the firewall (even ignoring the non-GUI related problems, which are also too many). Who ever thought out such a stupid and hard to use app rule editor is a complete moron. The thing should be completely binned and rewritten from scratch. Also, missing basic functionality, like jumping to related rule from a log entry, or copying the address from a log entry, or whois for a log entry and so on is just baffling.
And +100 for binning the registration, people are stupid, really, really stupid when it comes to computers. 99% of users simply don't have enough computer related thought capacity to successfully renew a registration. AND NO, THAT IS NOT A OVERSTATEMENT (taken from a sample of 70 people).