Yosemite One User Hangs After Login, Other Works
You have just duplicated the first part of the "why is my Mac crashing?" troubleshooting steps.

If it works fine under one user and the other user, not so much then the problem is not with the O/S it is something specific to the user.

Steps to fix this include booting into Safe Mode (hold Shift while booting) and logging into that account. Take the contents of that users Library/Preferences folder and put it in another folder on the desktop. and reboot normally. If that fixes it one of the many preference files was corrupt. If not...

Then there may be something installed in that user's account that is doing something bad. Go into System Preferences -> Users and remove the startup items for that user and reboot.

There are a lot of things that this could be. And it is a matter of finding what is unique to one user account that is crashing. It's a lot of fiddly, annoying investigations. It might be easier to just create a new user account, move only the necessary stuff to the new user profile and delete the old one.