For the most part, upgrading to OS X Yosemite didn't cause any issues for me, but the one change that bugged the heck out of me (and made me question what in the heck Apple was thinking), was switching the action of clicking the green icon in app windows. Previously, this would maximize the window on your screen. Now, it takes you full screen. (And this is where I question Apple. In all my time watching developers, I rarely see them use full screen. Heck, I used it for the first time last week for a video, but in my day to day work, never.)
If you Google for this, you'll see lots of people recommending holding alt-shift before clicking the green button. This works, but I've got the memory of a goldfish. Plus, in the same time I got my fingers set up to do this, I probably could have expanded the window manually.
Earlier this week I was attending a user group, and the guy next to me (unfortunately I don't remember his name), suggested simply double clicking the title of the app. It works! And it is easy enough to remember, at least for me. As a test, this morning I googled with that particular phrase, and sure enough, you can see this documented here at Apple: OS X Yosemite: Take apps full screen.
Anyway, I hope this helps others. As a quick note - Chrome seems to not widen in some of my tests (although I swear it did earlier). Holding shift makes it maximize that way too. That I think I can remember (maybe).
I only use the full-screen view when I have my laptop "undocked" from Thunderbolt monitors on my desk and then usually only for IntelliJ. Double clicking the title bar doesn't seem to work in all apps for me. For instance Chrome doesn't have enough above the tabs for me to really double click on. My RSS reader (ReadKit) doesn't seem to work either. However things like Slack, Sublime Text and Mail all work. So it seems like *if* the application actually has the title at the top of its Window, then it works. Still wish they'd left the full-screen behavior the way it was previously since the app title thing is kind of hit or miss.