You will probably miss the comfort of having everything just ready to work and working. Mac OS is a perfect OS, it's beautiful and it just works. But you have to follow Apple's rules.
Unlike with Ubuntu, you have to pay for most of the best software (although there are some open source or free alternatives for MacOS, too). Unlike Ubuntu, you can't choose your desktop environment. Ok, maybe you don't need to, the original one is quite nice. But it's a compromise between what Apple thinks you need and what you really need. Unlike with Ubuntu, where you can configure almost everything to your needs. You can even configure it to look like a Mac and behave almost like a Mac.
And I'm sure you will not miss:
- The need to buy a new OS each time a major upgrade occurs
- The feeling of having an outdated hardware, Apple is an expert in making you feel that your two or five years old computer is garbage and you have to buy a new one to have the shinny new things that the shiny new cat has to offer. And we are talking about Apple's own hardware! A wonderful and powerfull computer like a last generation PowerMac G5 won't run Snow Leopard! But you can run perfectly the latest (11.10) Ubuntu on a 10 years old power pc properly upgraded with processor and memory.
So, there is not much to miss
Maybe some software which only runs in Mac's, like OmniOutliner.
I'm a big fan of Mac OS but, honestly, with the development and growth of Linux, especially Ubuntu, Mac OS is becoming a useless luxury.