While developing the Foliopress WYSIWYG we decided to create the images management on the basis of Kae Veren's excellent KFM file manager. While we are totally happy with how KFM handles the images itself, we were unable to work with images uploaded via ftp.
Uploading images one by one through an image editor is fine, uploading twenty that way is annoying. One of the reasons to prefer WordPress over Typepad is that you do have direct access to the server via ftp. So this was clearly not acceptable. It wasn't even possible to change the file ownership of httpd via SSH (without root permissions).
One of the beta testers for Foliopress WYSIWYG has just complained that Foliopress WYSIWYG is not compatible with PHP4. Apparently PHP5 is still only 6% of the installed PHP base across all webhosts.
That figure should be enough to strike terror into any developer. But that number will change very soon as PHP4 has hit the end of the line.
PHP4 incompatibility started off not as a deliberate decision. Generally I like wider compatibility.
But on serious consideration, I’m not worried about Foliopress WYSIWYG being PHP5 only.