TBH I would very much recommend upgrading at least as far as Snow Leopard. There were a lot of performance tweaks in that version that should make your Mac leaner and faster - I had about 10GB more space on my hard drive once I'd done it.
There are also an increasing number of applications out there that have Snow Leopard as the base version of the OS in order to work - it seems to have been some sort of watershed release in the same way that Panther (10.3) was.
You need Snow Leopard for Firefox v4, and also for the Ikea Kitchen Planner plugin, which I was having much fun with last night.
For me though, the primary motivation for getting Lion is that I use MobileMe - actually, have used DotMac since launch, long before Apple decided to start charging for it. MobileMe is getting canned this time next year and the only way to keep my email addresses and other services is to move them to iCloud. And the only way to use iCloud on a Mac will be to have it running Lion.