I had the same problem installing postgres using homebrew on a freshly installed Yosemite.
First off my brew config looks like this:
HOMEBREW_VERSION: 0.9.5
ORIGIN: https://github.com/Homebrew/homebrew
HEAD: 9f6926265f8e4be7cc80dfe9042f2cd3c1e8dc9e
Last commit: 64 minutes ago
HOMEBREW_PREFIX: /usr/local
HOMEBREW_CELLAR: /usr/local/Cellar
CPU: quad-core 64-bit sandybridge
OS X: 10.10.1-x86_64
Xcode: 6.1.1
Clang: 6.0 build 600
X11: N/A
System Ruby: 2.0.0-481
Perl: /usr/bin/perl
Python: /usr/bin/python
Ruby: ~/.rvm/rubies/ruby-2.1.1/bin/ruby
First thing i noticed was that I had no write permission to /usr/local/var/postgres. This was easily changed issuing sudo chown -R `whoami` /usr/local/var/postgres then I reinstalled postgresql and did
cat /usr/local/var/postgres/server.log
which revealed:
postgres cannot access the server configuration file "/usr/local/var/postgres/postgresql.conf": No such file or directory
So I removed the directory /usr/local/var/postgres and issued the command to initialize the database.
initdb -D /usr/local/var/postgres/
This seemed to have done the trick and postgres is running fine.