I too found that I could no longer log on tonight after downloading the patch that downloaded when I logged on. I realize that having less than recommended hardware/software can often be problematic, but if something was going to change to the point of not allowing the game to even open it would have been nice to have some warning.
For some of us upgrading isn’t an option because of incompatibility with other systems we use. I often need to VPN in to my office and troubleshoot files from other users so my system must mimic theirs, keeping me from upgrading to Mavericks.
I will probably work something out so that I can play on another system, but that won’t happen tonight, or even this week, and maybe not this month. Lack of consideration for user experience and satisfaction is annoying.