There's a massive difference.
It would be a wall of text crit post.
There's a hell of a lot to do at 70 and it's nothing like being 99 in DII.
Some is pointed out here:
http://www.wowwiki.com/Activities_guide_for_characters_at_level_70
Gear does not really get less expensive, actually it's the other way around. But by 70 you are better able to afford it.
As to celebrating, there are multiple emotes to do.
http://www.wowwiki.com/Emote
Most notable are
/dance
/train
/cheer
/lol
etc....
You can also make up your own emotes.
For example if I targeted Kiba in game and typed:
/e puts %t's hand in a bowl of warm water and made him pee
the message would say:
[highlight]Gorny puts Kiba's hand in a bowl of warm water and made him pee[/highlight]