What am I missing in Mountain Lion?

I have a wonderful 2006 model Mac Pro, the original version 1,1, with twin 3.0Ghz Xeon 5100 CPUs and 12Gb of RAM. It has served me well, and continues to work tirelelssly without a burp. But now I need to move on to Lion to be able to be compatible with Aperture 3.4.3 and IPhoto 9.4.2 where I have my several other macs operating in OSX 10.8.2 Mountain Lion and sharing the single file with these new Apple imaging software versions. With many hundred gigabytes of images, I need the Mac Pro with its inventory of multigigabyte hard drives to help manage this.

I dutifully upgraded the Mac Pro's OSX in turn to every new OS from its initial OSX 10.4 before stopping at 10.6.8, like so many others, to sustain the use of a number of legacy applications. I now have an external boot drive running 10.6.8 which will boot my MacBook Pro, my IMac, and my Mac Pro and run these legacy applications.

So it is time to upgrade my Mac Pro to Lion, OSX 10.7.5. Unfortunately, this Mac Pro 1,1 cannot be upgraded to Mountain Lion.

So my question is what functionality or interactivity will I be missing on my Mac Pro 1,1 runniing OSX 10.7.5 while my other Macs all run Mountain Lion 10.8.2?

All of the reviews of Mountain Lion I have read list new functions over Lion which seem somewhat trivial, but I want to be certain that I will not be missing any core functionality, or at least that I know it going in. The use of ICloud seems to have been enhanced, but very much present in Lion. The required OS for the latest versions of Aperture and IPhoto require 10.7.5, which will be covered on the Mac Pro. These OSX reviews are somewhat difficult to discern asking the backwards question of what compatibility issues I may face running one Lion machine, and what functionality I will be missing in Lion.

So I am looking for some practical experience from anyone who has continued to run Lion with other Macs running Mountain Lion. Any help or experience will be welcome.

BTW, I am anxiously awaiting a (soon to be released?) new Mac Pro model to which I plan to upgrade so soon as it is available. I need the tremendous expandability the Mac Pro concept has offered over the years, while all of the other Mac models have essentially no remaining expandability.

Thanks!!!