Sunnyjohn,
Apple give you 200 reasons on their website
here.
Fundamentally, yes, the major headline changes are the social media and sharing stuff.
However behind the scenes, there have been new APIs introduced and improvements to existing APIs. It will of course take a little time for all the third-party software developers to adopt these.
I would say, if you are on 10.7.4 now (i.e. 10.7 with all the current updates), then no need to rush, upgrade when you're ready, but don't wait forever (I'm saying months, a year max.... i.e time for a few 10.8 dot releases to happen).
Apple haven't completely stopped development on the 10.7 branch, just are not introducing new features, just tweaking and fixing existing , there's a 10.7.5 update in the pipeline focusing on graphics performance and quality (can't tell you more than that... since its under Developer NDA).
If you are not on 10.7 now, then you might as well go all the way up to 10.8.