You should be in the 10.6x range...10.8x isn't out yet. Haha. Did you try running system updates to make sure you're up to date? They recently released a Graphics Update for OSX that is required to play L4D2. Click the Apple in the top left corner of your Mac and click About This Mac to see exactly what you are running. I'm running the game perfectly on my early 2008 iMac.
It's a little different than the Windows updating process, which has a major release then subsequent service packs (ie XP, Vista, 7). Mac OS 10.0 came out about ten years ago and has been adding a tenth each time, but normally they include Windows level of changes for $130 upgrades, which is close to Windows retail upgrades. Snow Leopard was more like a $30 service pack, but I would say that it was almost like somewhere between a Windows SP and a full release so it was worth the extra $30. It's difficult to compare Windows and OSX updates because they are handled differently. Just like everything else about the two systems. It's like people saying that Windows 7 was just really Vista SP3. That's not really a fair statement. Win7 is great, not AMAZING!, but great.