Not criticizing you, Matthew. But it's very likely that it is Mavericks, not anything Google did.
For others who might come to this forum, let me summarize a few things, so that people won't keep repeating the same "solutions" over and over again.
1) This problem has been happening on many different devices since the Mavericks update to 10.9.0. It is not unique to the Galaxy, or Samsung phones, or the like. Although different phones may be having different problems. My phone is a Motorola Droid RAZR HD, and it was working JUST FINE until the Mavericks update. If your phone is having problems with a different version of OS X, try some of the simple things below.
2) Anybody having this problem should first go into their USB settings and make sure that the USB mode is "MTP". Also make sure that USB debugging is off in Developer Options if you have that enabled. A few people have reported that it ONLY works for them when USB debugging is enabled, but they seem to be a rare few.
3) If you have a Samsung phone with KIES installed, what you should do is download the KIES installer, and use it to UNinstall KIES. People have reported that simply deleting the app doesn't work. Be sure to restart after uninstalling KIES.
4) If neither of those work for you, you can use Airdroid or another alternative to transfer files via WiFi. (There are actually many apps that do this, and I prefer some of the others over Airdroid. But you also need to be pretty network savvy to use some of them.)
5) HOWEVER... and this is an important point: for the rest of us, this problem is still there and hasn't been fixed. And we DO want it fixed, because WiFi is very slow compared to USB transfer, and quite a few other issues. And yes, for most of those people it IS a problem with OS X Mavericks. I know because there is a workaround for some phones. But you may not like it:
Connect your phone using USB. (As usual, make sure the mode is "MTP".) Open Network Preferences. You should see your phone showing in the list of devices, even if you're not communicating with it.
If so, then restart the computer with your phone connected. (Some people say they have to uncheck "Reopen windows when logging back in" for this to work.)
Once you restart, Android File Transfer should start and you are connected! But be warned that if you unplug the phone, Android File Transfer will stop working until you plug back in and restart the computer!
So for many people, WiFi transfer is the only practical option.
But this is why I say: except for those people who can fix this with the simple solutions above, the problem for the rest of us *IS* a problem in OS X Mavericks. If it weren't, this workaround would not connect you.
6) And last but not least, the solution at the following link has worked for many people,
http://trjlive.com/2013/10/guide-android-file-transfer-mac-osx-mavericks/
But it doesn't work with my phone, and may not work with yours. I'm still waiting for Apple to fix this, and I am very angry that they haven't. It is unconscionable that this is not fixed after at least 4 months of continued problems and complaints.