I'm having a weird problem that just started with my 2012 15" Retina Macbook Pro with Yosemite. The ability to tap and drag using the internal trackpad has stopped working. All the other gestures work just fine, including three finger drag (when I enable it - it's off by default) and two finger scrolling.
I can perform the gesture just fine using a wireless Magic Trackpad.
I've tried both SMC and PRAM resets and nothing helps. Anyone seen the same issue?
It seems like apple moved the option for enabling tap-to-drag to the accessibility pane. You can enable it there:
Note: ticking three-finger drag (in the Trackpad prefs pane) will change the Accessibility-pane setting. Unticking the drag setting in the Trackpad-pane first then follow these instructions to check your Accessibility preferences.
I had just done some testing to check this out when answering another question before I found my way here. That ability, at least on the Bluetooth Multitouch Trackpad, seems to be gone, at least for now. See the other answer if you want a few more minor details.