It appears that the problem is rooted in the SyncServicesAgent. The SyncServicesAgent hangs on startup preventing the Outlook window from appearing.
For those using, multiple IMAP accounts, the problem resolves itself when removing the accounts as Outlook isn't forced to perform a Send/Receive and sync IMAP folders at startup. As POP and Exchange use different transport mechanism, it appears that Outlook starts up as expected although I suspect that those syncing with mobile devices via iTunes may discover that Contacts/Calendars/Notes are no longer being synchronized.
For those using Outlook & iTunes to sync mobile devices (i.e. not iCloud, MobileMe, etc.), navigating to the Outlook > Tools > Sync Services menu will display a window that:
a) is blank, no services present, or
b) services present but Calendar, Contacts will not sync to the device and Notes is not an option under iTunes > Device > Info
Understanding that the current "workaround" allows access to Outlook, it is temporary and not an acceptable solution. As per previous posters, it is likely that Microsoft will need to regression test the code from the last update against the current OS X (10.10.3) updates and release a fix.
Cheers,
hgagne