The conversion to PDF Kit was the right move because Apple continues to develop PDF Kit and TeXShop then automatically inherits their improvements. This conversion also marked the end of TeXShop's 1.** system of releases, and the beginning of the 2.** releases which only work on Tiger and higher systems.
There is one very significant bug in Tiger's PDF Kit implementation. That bug causes significant memory problems when a pdf document is changed and the display must switch to the new version. Of course, TeX often changes the pdf output as source files are retypeset.
On Tiger, TeXShop has some "band-aid" code to work around this bug. That code causes TeXShop to retain old versions, gradually using up system memory. The PDF Kit bug was fixed in Mac OS X, Leopard, released on October 26, 2007, and TeXShop automatically adjusts and removes its band-aid code. For that reason, the TeXShop web site for many years has stated that "System 10.5 or Higher Strongly Recommended."
I'm sure Horn was frustrated, too. He switched TeXShop to use svn, an entirely reasonable change. But I found svn impossible to use. Occasionally users would suggest a TeXShop improvement, I'd add the improvement, and then do battle with svn and the improvement wouldn't be released. Later I began releasing versions of TeXShop that weren't in the svn and the entire process fell apart. In the end, Horn wasn't able to complete his revision because someone (me) wasn't cooperating.
I tell you these details to explain that the life of my collaborators hasn't always been a picnic. Collaboration has worked best when a developer submitted code with changes clearly marked, and allowed me to select those changes to include. More professional collaboration, of the type used for most open source projects, hasn't worked because the guy in charge, namely me, isn't competent enough to adjust. Apologies all around.
After Horn's changes were released, in version 2.10, other collaborators again began advancing TeXShop with new localizations, improvements in the Key Bindings and Auto Completion editors, improved handling of matching parentheses, and many other changes. Consult TeXShop's version history web page for details.
Since the release of Tiger, TeXShop has not made use of operating system advances except in minor details, and version 2.43 still runs on Tiger. Of course Apple deliberately didn't change the interface in Snow Leopard, concentrating instead on fundamental changes in the underlying foundation of the system. One opportunity these later systems afforded was the ability to rewrite programs for 64 bit operation. I wasn't convinced that this change would improve TeXShop and didn't switch it to 64 bits until very recently.