1. How are you connecting to the internet and to WordPress.com? The way you connect to the internet (mobile, satellite, DSL, dial-up) and to your blog, and how many proxy server jumps etc. it takes to connect can cause problems.
2. Are you using http:// or https:// see http://en.support.wordpress.com/https/
3.Exactly which browser and version of it are you using? If you don’t know click here to find out > http://whatsmyuseragent.com/
4. Have you tried clearing your browser cache and cookies? http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=32050
5. Is your browser version up to date? You can upgrade any browser version here. http://browsehappy.com/
6. Have you tried using the browser with all extensions or add-ons disabled?
7. Do you have third party cookies enabled? http://support.google.com/accounts/bin/answer.py?hl=en&answer=61416
9. Do you have another browser you can try?