Whether or not you're "okay" with Age of Sigmar will really depend on what drew you in to Fantasy and not another popular game like Warmachine or Infinity.
Age of Sigmar is a "skirmish game". There are no more big blocks of 50 models. In fact, there are no more "blocks" of models at all, as everything moves in loose formations just like in 40k, Warmachine, or Infinity.
The lack of a points/balancing system is not a
huge deal, actually. The game is decently balanced "as is" and will get better as the old armies are phased out* and the new WarScrolls replace them. What it
does mean, is that the tournament scene has had a major shakeup, because we're left to find our own way to insure that players don't "break" the game.
In friendly group play, the players are meant to moderate themselves. If Chad is a jerk that wants to put 15 dragons on the table and wreck everyone, then don't play against Chad. Chad will either change his mind, or stop playing the game. This works in small groups, but if you want to play a "competitive game" where winning and losing matters more than some ambiguous notion of "telling an interesting narrative" or whatever 'hippy nonsense' GW wants to spout your way, then you'll either need to find community support for balancing AoS (there's a lot already) and agree to it club-wide as being just as "valid" as the GW rules,
or you'll need to find another game**.
The nitty-gritty details of the game probably won't phase you much. They've made it
very streamlined. This is
good for a new player, because it means that instead of reading 120 pages of rules and probably having to reference the core book at least once each match (even when you're me, and have been playing regularly for over a decade) and
constantly flip through your armybook, you only need to read
four page of core rules and then everything else you need is contained right there on your War Scroll cards that are sold with all new Age of Sigmar models,
or are available for free as pdfs from GW's site.
You have a 40k background, based on your sig, so it's worth noting that the traditional format for statlines is gone. There's no more WS or BS, for example. Your card gives you a specific "To Hit" roll for each attack, and 'To Wound' roll. It doesn't matter
what you're swinging at, you will
always hit on X+ and wound on Y+. There are no modifiers to these values as part of the core rules (Fantasy used to have 'To Hit' modifiers for shooting, to account Range, and armor was modified by Strength) but there
are certain units with rules/spells/abilities that give a +/- to certain rolls.
Also, units do not Break or run towards the board edge anymore. You just remove models for every point your fail your Leadership by. This is
super important to the game balance. I've seen a few people try to ignore this rule or balance around it, and it's always the first place that they go wrong.
In regards to "fluff", GW blew up the world and everyone is dead. That's basically it.
Sigmar survived, and found a new world that was completely overrun with Chaos. "Orbiting it" (the best phrase) are a collection of other realms which Sigmar has repopulated with the races of the former world (but not quite). These realms are all connected with Warp Gates so that people can get around 'Super Mario Bros' style in order to have unnecessary conflict in order to give players something to do. Think about it: if we discover life on Pluto, Earth isn't going to suddenly put aside all of our "petty" differences like
Race, Religion, Natural Resource Management, Gender Equality, who-cheated-to-get-the-shiny-pokemans, etc and decide "let's go fight the Plutonians!". But that's what the Realms are doing. Oh well.
The new races
aren't the old ones anymore.
Elves are now Aelfs, Orcs are now Ooruks, Lizardmen are now Seraphons, etc. Because those names weren't generic or public domain, and can therefore be copywritten and
now nobody can market "Elf Spearmen" and undercut GW's sales because
how would anyone be able to figure out that Elf Spearmen can be used as Aelf Spearmen?! (yes, they think you're that stupid, it's GW, get used to it) (also, don't mention how similar Ooruks and Uruk-Hai sounds. Just don't. GW is
original as fu** kids, don't you forget DA REAL O.G-Dub!!)
Sigmar is now basically filling the role that Chaos used to. He pops into reality and wrecks stuff, then heads back into the warp. GW just thought that it might be more PR friendly to say that their most supernatural, "traditionally God-like" entity in the game has more in common with Angels than the assembled legions of Hell. Timmy's mom is very happy with this, and will now let Timmy buy the game because it no longer promotes Satanism in the vulnerable youth of today (again: GW judges us all by the
lowest common dollar-generator)
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*The old armies are getting whacked. Totally and utterly. GW has confirmed that it will be phasing out the current model line at a more rapid pace. That's totally normal, as old minis were always updated by new ones
eventually (or you got them in Finecast because it's cheaper for GW and they can put a higher price tag on the new packaging). However, these updates always stuck to the same "aesthetic" as the former models. Not anymore. Empire and Brets are getting phased out to be replaced with the "SigMarines"-looking faction. If you liked the historically based Humans, or the Viking-inspired Chaos Warriors, that's too bad. Everything is going to look like concept art from a rejected WoW expansion now. Unless you're cool with that. In which case, good for you!
**Other Game
Right now, Kings of War is looking
real sexy in comparison to Age of Sigmar,
if you liked Warhammer as it was. The fluff is obviously
completely different because that stuff is all copywritten to hell and back. However, the same armies are all present (or will be soon) at least in spirit, most of the units are direct ports (why GW is ditching the old stuff: they essentially let Mantic win that fight) so you can use WHFB models that you pick up cheap on Ebay instead of the much cheaper but less pretty Mantic models.
The game rules are still very streamlined, ironically they're more similar to Age of Sigmar
except that Kings of War
does have the models ranked up in gorgeous block-regiments, and
does have points as a balance mechanic. In fact, KoW was written with the express intent of being played as a tournament game. The balance isn't
perfect (never will be) but Mantic is doing a very good job of supporting the tournament community (GW doesn't do this
at all) and communicating directly with their players (GW doesn't do this either) to make sure that things remain at least
reasonable. Oh - and Kings of War was developed by Allessio Cavatore - one of the guys who did some of the earliest design work for Warhammer Fantasy.
Personally, I'm
not getting into Kings of War, but I
do play Age of Sigmar. I'm an "old head" and I refuse to switch systems. I will continue to play Fantasy is some guise for ever, even when it's no longer supported and my modes are slowly disintegrating into plastic shrapnel. What I
am doing, is working on updating Fantasy to a "fan generated" 9th edition. Other people online are doing the same, and other gaming companies are starting to tentatively test the waters on how much blatant plagiarism GW will let them get away with.
If you're familiar with Dungeons and Dragons, expect to see a situation similar to the 3.5>4th + Path Finder split. Wizards updated to 4th Edition and everoyne
hated it, and another company created Path Finder as an
exact clone of 3.5 (this was legal because Wizards actually made the rules to 3.5 public property, and then Paizo took them, made Pathfinder, and copywrote it all for themselves anyways). Path Finder almost killed D&D - as in, Wizards halted production on 4e entirely, and almost didn't release 5e.
This is likely to happen with GW.
Here is
my "pathfinder" design thread for Warhammer. My game is quickly panning out to be very "7.5 edition" actually, being more a cross between 7th and 8th, with a few new mechanics and a focus on streamlining:
http://www.librarium-online.com/forums/rules-development/302642-thoughts-8-5-edition-army-updates-2.html#post2740049
