As for every tranactional database, disk I/O is the main limiting factor for PostgresSQL. If you plan to deploy a high-usage database please take some precautions on the used storage. Use either a RAID 1 or 10 and choose a FS that does fast block-IO (ext2).
Accounts and permissions
When starting with a fresh database, you've to create some users first before you can start using it.
su - postgresql createuser -P <user> createdb -O <user> <db>
That should settle it.
Also be sure to tune the pg_hba.conf in the data-directory to your needs. To emulate MySQLs default "every user needs to authenticate from everywhere"-semantics use the following config:
local all all md5 host all all 127.0.0.1/32 md5 host all all ::1/128 md5
Since PostgresSQL is a transactional database, old rows don't get actually removed/replaced when you update/delete them (since they might be still needed in older/long running transactions). To actually free them you need to issue a vacuum.
A normal vacuum will only mark deprecated rows for reuse, to actually reclaim diskspace (e.g. when having deleted large amounts of data) you need to issue a full vacuum. Please note that it might be faster to backup the data you want to keep and truncate the table if you plan to remove large portions of a table.
Fixing broken databases
set zero_damaged_pages to on; vacuum; pray;