I think you can do some of the fancy gestures with a Magic Mouse, and of course you don't *need* to use the gestures. But Mission Control, gesture to reveal desktop, swiping between virtual desktops/full-screen apps, etc, is all very nice. Lion also has a number of under-the-hood improvements (like security,
http://www.theregister.co.uk/2011/07/21/mac_os_x_lion_security/).
I say this, because I've been running the developer betas at work for a while. Some of the animations that happen when you drag-and-drop things bug me, and iCal and Address Book are too
skeuomorphic for my taste.
You don't need to upgrade until software you want to use starts requiring that you do, of course.
I've recently upgraded my home computer from an older iMac to two computers, a new Macbook Air (got yesterday) to actually do work on, and a giant, ridiculously powerful PC running Windows 7 (to store media, back things up, play violent games, etc). I like this setup a lot.