i am speaking from experience. and without getting into it myself, please refer to some others' opinions.

http://milesj.me/blog/read/wordpress-is-bad-mmmk

http://www.usingwp.com/general-WordPress/why-WordPress-sucks...

http://www.phpvs.net/2009/12/08/an-exercise-in-WordPress-int...

http://premium.wpmudev.org/blog/7-bad-things-about-WordPress...

http://stackoverflow.com/questions/1481631/why-is-wordpress-...

http://someguyjeremy.com/blog/why-i-hate-wordpress

http://bergie.iki.fi/blog/wordpress-decoupled/