The Shattering has happened and the world has changed. One of the biggest changes is new starting areas for all the races except the Draenei and the Blood Elves, which had a couple of small tweaks mainly dealing with the class trainers and the addition of a trainer the can teach all the professions.
Here are the my impressions of the Level 1-5 areas:
Orc/Dwarf/Night Elf - The quests flow smoother but all in all, very similar to the original areas. Orcs are introduced to conflict with the humans early on here. Big change for the Night Elves is the Moonwell quests actually have a spirit show up and tell you the history, instead of you reading it.
Human - Completely redone, brings to the fore the conflict between the humans and the orcs.
Troll - Very well done. You can background on the retaking of the isles talking to some of the NPCs, as well as facing the Sea Witch from WC III, with Vol'jin before heading to Sen'jin village.
Gnome - The other new starting area, it is also well done. Mekkatorgue shows what happened at the end of the retaking of Gnomergan, and the story picks up from there. The hardest part is facing the boss at the end of the quest zone before heading Kharanos. It helps to have friends to deal with the mobs that spawn as you use Mekkatorgue's special abilities.
Tauren - You get to see of the changes brought about by the Grimtotem attack, as well as a seemingly larger Quillboar presence. The quests go lot a faster, and the overall size of the zone feels smaller.
Forsaken - While several of the quests are similar to the orginal quests for the area, the story is updated to reflect the changes brought about by the fall of the Lich King, and Sylvanus's recruiting of the Vrykul. You get to see that several of the newly risen dead are not accepting of their new state, leading to them become mindless undead or forming groups that stand against the Forsaken. Excellent storyline here.
Take some time to check out the new starting areas and see the new lore. Also the equipment you receive from the various quests, have a more uniform appearance, looking like the belong together.