I am also a pure AoS player so I might be not the best person to answer that question, but here are some big differences from the top of my head
- AoS has a much more simple ruleset. The whole basic rules of AoS are not even the size of the WHFB rules covering movement of flying units. That has both advantages (such as being WAY more accessible for new players and making the games faster in some regard) and disadvantages
- new lore of course. Some people dislike it and prefer the old WHFB because of that. Others just ignore parts of the new lore they don't like or actually like it.
- WHFB works only with square bases, in AoS you can use square or round (tendency goes to round though, and IIRC some tournaments even require them)
We have one or two AoS vs. WHFB threads here on the forums, but be careful reading them, it is a very hot topic for some users here.
Here is the most recent one:
http://www.lustria-online.com/threads/fantasy-vs-aos.20517/
Tomb Kings look great, they are egyptian undead.

They also have a very complete set of units covering everything from Infantry, Cavalry, Monsters and Wizards and even Artillery.
EDIT: Oh, and pretty epic stories in the old WHFB lore or at least that's what many people say.