I've found application boot time to be a pain point if you're running your tests frequently - I run mine several times a minute.

For Ruby/Rails developers it's been especially bad, especially in JRuby. Some major patches have been made to Ruby(MRI) recently to help make this less of a problem:

http://www.rubyinside.com/ruby-1-9-3-faster-loading-times-re... http://blog.tddium.com/2012/11/20/profiling-ruby/