As I, and many others, have explained countless times OSX uses your available memory for caching. The more you give it, the more it will use.
In fact, Windows 7 manages memory in a very similar way to Lion. Page outs not free memory is what is important.
Actually OSX doesn't work the way Windows 7 works. Windows 7 actually caches with a smart algorithm behind. OSX works much like Windows XP used to it just keeps stuff in memory that is not used anymore in case it is used again. XP did the same it just never reported it. Free to use was free to use.
My comparison between 2 GB win7 and 8GB SL is not really fair because I used the 2GB Win7 a couple years back with applications that also used less although Eclipse memory use didn't grow all that much. But there are so many apps that used to need only a few MB ram and now need close to 100 like mediaplayers, VLC even. Everytime I compare that to Windows the Windows apps gets away better.
It is also just not true that useing RAM if it is available is always the best strategy. Most of the times it is just lazy programmers and frameworks used the wrong way or.
I am just programming a little thing now too that if done with the some care needs very little ram without ANY performance problems but with a few bugs that you don't fix you end up with some RAM sucking monster at least compared to the work it does and the little data it need to actually hold in ram to work. Compressed that would be no more than a few KB.
Some stuff like databases really benefits from more RAM use but if you use the RAM where it isn't really needed you take it away from stuff that might make better use of it. As long as there is such a thing as too little RAM it is something to complain about.
Browser got a lot more RAM heavy but they did give some speedier at rendering. How you hold your data in ram and access it makes a difference.
With some Browsers like Opera you may at least set a threshold for RAM use. Chrome just keeps spawning processes until no RAM is left.
The whole stability issues also need RAM. When some stuff used to be shared between tabs it takes less RAM if every tab spawns its own process it needs more RAM that is just how it is even if it gets you nothing at all speed wise.
OSX just gets away with it but if Windows was such a resource hog people would complain all over this form how bad Windows is. I think the Win8 feature with low priority RAM is a really good idea. Maybe OSX gets it someday too. Though the benefit still hugely depends on 3rd party developers.