I am one of those who absolutely LOVE wordpress. I have worked a lot with it and I am used to the way it works. It probably have a lot of shortcomings, I can't see them, but I'm sure you guys do. Would be interesting to hear them. 
There is a saying about the right tool for the right job. So I'm sure Zend kicks WordPress ass where there is special needs, but it wordpress easy to use and deploy but the same time have some advanced features. For most web pages wordpress does the trick and if you need some extra stuff you could always make a plugin.
It's probably just me, but I feel that doing something custom with Zend require a lot of work and then there is the thing about making yet another cms/system/whatever it might be when theres a lot of good alternatives. I guess I would understand if it was for the challenge or something like that, but that is probably not the case in an enterprise environment.
One very defining thing is who difficult it is to work with. I'm pretty sure every novice PHP developer could understand wordpress relatively easy and start messing around with it. Maybe even make plugins. The learning curve for Zend is a lot steeper (is my experienced at least). I feel like come from the opposite place of you, iznogood. Going from WP to Zend and you the other way?
I have no need to hide that I am a wordpress fanboy, that being said Zend looks awesome and want to get to know it. At the moment, for me wordpress and Zend isn't competing. I feel like they cover different ground.
But yeah, the part where all the stuff you have changed disappears on update, is not cool.
What do you mean with "The fact the client can destroy everything if he wants from his cms. " Are you thinking about the editor?
This post became a little longer than I initially thought it would be 
