I can't say if it's the case for Lion, but certainly for database servers, the reason that you only want to use 60% of the hard drive space (with traditional disks, not sure about ssd's) is as using more than this results in the disk head having to move further to read the data - resulting in slower IO times.
As a result, in order to maximise the IO throughput you want to minimise the fragmentation of files on the disk and stay away from the longer tracks on the outside of the disk which drastically reduce the disk IO.
So assuming that this affects lion (which is fair enough as it affects all traditional disks), you'll notice a drop off in the disk speed, and so a reduced user experience, from a nearly full disk.
Hope that makes sense, as I say I'm not sure if this is the case for lion - but I'd be shocked if it didn't suffer from this as all physical disk systems do. All that changes is the percentage that it becomes obvious and that is dependent on how the disks are used and not the os that's being run.