With Yosemite(at least with the default security settings), Apple seems to be trying to make OS X almost a walled garden like iOS. Any time I want to install something not downloaded from the App store, I have to go into system preferences and specifically allow that program to install.
It's a pain, and is one of about 100 reasons why I'm not running Yosemite on a computer I actually depend on...
I work with a bunch of recent Mac converts, most of whom have zero experience with anything pre-10.7. Lion introduced two big changes in the default UI-the Launchpad, and it defaulted the finder to not showing mounted volumes on the desktop. The launchpad, of course, is easily ignored if one so desires and showing mounted volumes on the desktop just takes a couple of clicks to enable(fortunately, even Yosemite still allows this).
10.5 is a totally different experience for most of these folks, however, as it seems strange to them to go to the applications folder to launch an application rather than the Launchpad. It drives them crazy when they try to use my G5 for that reason(but also keeps them away, which is a good thing). At the same time, however, the lack of volumes on the desktop drives me crazy when I'm troubleshooting, as all of my computers(including my MBP running 10.9 and Macbook running 10.10) show them.