I'll just echo some of their sentiments.
Yes, it's a good to great Total War game, if you don't mind fantasy elements thrown into your Medieval warfare.
There are currently fewer overall factions, but they are much more unique. Very few units are shared across factions.
No, you do not need to know anything about Warhammer to enjoy this. I would say that they follow some typical fantasy tropes, but that does a disservice to Warhammer, which arguably CREATED a lot of the tropes.
Empire of men, who generally just wants to spread the influence of man as widely as possible, but is fractured by infighting.
Vampire Counts, who are not actually inherently evil. Mostly just pissed off at the Empire for past conflicts, and would be perfectly happy living in tandem with normal Humans. Main goal is to restore their brethren, and overthrow Karl Franz as Emperor.
Greenskins, a savage mixture of races who simply love to fight, and will happily go to war with anyone nearby. Their goal? WAAAAGH!
Dwarves, who share borders with the Greenskins, and absolutely hate them. Fairly tolerant of everyone else, as long as you don't piss them off. Literally hold grudges for generations. Goals? Crush Greenskins, protect their borders, and settle grudges.
Chaos, who are basically corrupted members of all the other races, hate everything and only value strength. Infighting encouraged so the strong rise to the top. The only truly evil group. Goal? Destroy everything they can't convert to Chaos.
Beastmen, who are chaos influenced. I know the least about them, but they generally just seem pissed at anyone who is destroying the wilderness they live in, so Men seem to be their main enemy. Goal? Crush anyone would invade their wildlands.
And very soon, Wood Elves. I don't know much about them. I assume they're similar to Dwarves, in that they just want to be left alone but share land with Beastmen.