Patch 5.0 includes all of the changes that will be available for players who do NOT purchase the expansion. (Not all of it is on the PTR yet, though.) So it's a good way to see what's going to happen to you no matter what.
- Account-wide Achievements, pets, and mounts
- New UI for mounts and mini-pets
- AOE Looting
- World Event: Attack on Theramore Isle
- New talent system, updated glyph system, improved abilities, and spells; new spells are now learned automatically
- And quite a bit more. Here's the details from Blizzard: http://us.battle.net//wow/en/blog/6761563
Edit: I *think* the mini-pet battle system is available on the PTR, but I want to get confirmation before I list it.
For hunters specifically:
- Ranged weapon slot removed; ranged weapons no longer have a minimum range
- New hunter pet talent system (http://www.wow-petopia.com/pandaria/talents.html); pets can choose from any of the specs (Cunning, Ferocity, Tenacity)
- Hunters can tame basilisks and fen striders (in the water strider family)
- Some pet family abilities have changed (http://www.wow-petopia.com/pandaria/abilities.html)
What's not included:
- Access to any of the Pandaria zones
If you want to take a look at Patch 5.0 on the PTR, start here with Blizzard's PTR FAQ: http://us.battle.net/wow/en/forum/topic/6146565384#4