There was a TON of controversy when Age of Sigmar released (and rightfully so). For a start, the new version of Warhammer Fantasy changed dramatically. It went from being a rank and file strategy miniature game to a skirmish game. This left veterans feeling like being slapped in the face. From a modeling perspective, it took awesome set pieces people made based on how the game was originally made obsolete overnight. While yes, you can still play with the square bases, it was not the intent of the game overall in the long run. It wasn't gradually introduced and given feedback from veterans. It was a sudden introduction at a time when Warhammer Fantasy is not the greatest selling product on Games Workshop's library. No restrictions or point structure made the game feel more geared towards taking powerful hero characters or monsters without need for the units. However, I did want to point out some good things that came out of Age of Sigmar as well as changes that made it better over time.
First, one point that is often overlooked is that the rules themselves are free to download, even for the units you put on the table. Free, as in no money is required to buy them. Even in the army books that are sold, the basic rules are always in the back of the book, four pages long in fact. Don't believe me? Look here.
Go ahead, I'll wait.
This was a pretty dramatic change as well for Games Workshop, who traditionally sold rulebooks in order for people to play their miniature games. Now, they publish the rules for their miniatures online for free, even new releases like Alarielle the Everqueen. This does a few things for the company. For a start, it lowers the barrier of entry and makes the transition that much easier for veterans. Before, in order for you to play you not only had to buy the miniatures but you also had to buy the rulebook associated with the game and the army you were playing. Now, just get the miniatures, put them together, download the rules for free online, and your good to go. This is due in part to competition coming from companies like Privateer Press and Wyrd Miniatures making their rules available either free or for an extremely small fee. Warmachine and Malifaux miniatures come with a card that details the rules of that miniature you purchased, making their army books completely optional. Warhammer Fantasy rulebooks were close to 75 USD retail value versus Warmachine where the rules are available online for free via an app. Warhammer 40,000 has had a bit of a army book crisis lately. There are a total of 30 Armies in the Warhammer 40,000 library. Meaning that if you wanted rules for all the different miniatures Games Workshop produced, you MUST pick up a copy of the corresponding rulebook, an average of about $40.00 per rulebook. This isn't including the supplement variants that have come out recently like the Gathering Storm or the Champions of Fenris. Having people experiment and try different armies was not only time consuming but also a heavy investment on top of the miniature purchase. Now, interested in trying something else? Just download the rules online and try it yourself before you buy it. This is a positive change that I hope we see more of in Warhammer 40,000 going forward.
Second, the rules themselves are incredibly easy to pick up and learn. All of the corresponding rules are broken down into 6 phases: Hero, Moving, Shooting, Charge, Combat, and Battletest. Although I do think Charge and Combat should have just been combined, but that's just me. Models have a movement statistic that shows how much in inches each model can move. Each model has a certain amount of attacks that need X to hit, say 4 attacks at +3. No matter who you are attacking, their position on the battlefield, rain, snow, sleet, or fog, it is always the same number. The same goes for wounding. Saves are this time around modified, so that a certain attack will yield either no penalty to the save or a rend modifier, which lowers the save. So if you get hit by an attack that is -1 rend, your save of 3+ on the dice becomes a 4+. Pretty simple right? Casting spells is done by simply rolling 2D6, adding them together and seeing if you meet or beat the target number, like 6 for Mystic Shield. Dispelling works the same way, but only Wizards can dispel one per turn unless otherwise notated. Magic is done during the Hero phase of the game. There isn't a numerical advantage to having an army of Wizards anymore. While this does turn a game of tactics where flanking makes a huge difference in moral and can turn the tide in your favor to a game where it is nothing but heavy metal mosh pits, it does speed up play quite a bit. Initiative, or who swings first in close combat, is based on who's turn it is. Games can be completed fairly quickly compared to other Games Workshop titles. Warhammer Fantasy is a game that can last up to an hour at minimum and on average was about 2-3 hours to complete. The reason for this was in the complexity of its movement, shooting, magic, and close combat phases. Age of Sigmar can be wrapped up in about an hour and a half max if you know what you are doing with an average sized game.
Previously, Games Workshop released Age of Sigmar without a point structure. This meant that people basically had to come up with their own method of a fair and balanced game before the game started. With the release of the General's Handbook, this introduced a system where players can either play narrative battles, match battles (where each player is given a certain number of points to spend on units), campaigns, and open play (which is essentially no restrictions). The point structure is far simpler than it used to be. Previously, each model had a point cost with heroes having a laundry list of magic items and options to choose from. Now, each unit has a point cost and goes up in increments. For example, Skeleton Warriors are 80 points for 10, 160 for 20, 240 for 30, and so on with no maximum on the size of the unit. Items in the unit like standard bearers, musicians, and champions are free and have rules associated with their unit. They kept the minimum requirements so that you have to take at least a certain number of units that are Battleline, which are the normal "rank and file" in an army. Models like special characters such as Nagash and Archaon are a limit of one per army. Heroes and Behemoths like giants and dragons have a restriction on how many you can take based on the points agreed upon. Some Heroes even count as Behemoths and do count as one of each for army selection, so Nagash is a Hero and Behemoth, meaning he counts as both a Hero choice and a Behemoth choice. If you restrict yourself to taking an army out of a specific type like Corpse Eaters, it makes Ghouls count as a Battleline for your army. Each army gets an artifact that is rolled on a table before the game starts. These are basically the new equivalent of magic items, but are free to take and don't cost any points. Each army gets only one of these though.
Weird, sometimes game breaking rules, have been gutted from Age of Sigmar. Rules like Psychology, Lance Formations, Animosity, your Undead army crumbling should the General die all were removed. Most of the rules related to armies are not on a universal basis but rather by a unit basis. For example, Skeleton Warriors no longer go past their maximum starting size throughout the course of the game and can only bring back D6 Skeletons a turn up to their maximum when starting without the need of a Wizard. They still suffer the same effects from Bravery like other units. Morale is done by simply removing models for each point you fail by when making your Bravery Test, which is done at the end of the round. Raising new units in Matched Play is done by setting aside points for them to be summoned instead of just simply summoning new units on the table for free and having a number advantage over your opponent. In the few games I've played, I never noticed a situation where one side had an obvious advantage aside from the one guy who took nothing but giants. The rules themselves for most of the armies I've seen haven't been too overpowered or broken and even the ones that were got rules changes pretty surprisingly quickly considering Games Workshop has had a history of long and strange decisions when it comes to the rules...
However there is one word of caution I do have to advise. The game itself was designed with 32mm bases in mind, not the 20mm square bases of the past. Meaning that you actually have an advantage using the older bases versus newer players who are using the round bases provided in the package. You can pack in more models around with square bases as it was naturally designed this way. Combat is no longer resloved base to base but rather by reach distance. Most models have an inch of reach in melee, while some have 2" or 3" depending on the weapon. Distance is measured from the model to model, not base to base in this game.
If you do decide you like Age of Sigmar and have a current group going at your LGS, I do highly encourage you to convert them over not to have an unfair advantage on your opponent.
If you have some Warhammer Fantasy models lying around, I'd give it a shot. The rules are free online and it'll take about an hour of your time to get in a game. Do some basic models and don't include special characters or Behemoths as of yet. Just a general and some units is all you need.
Give it a shot and see for yourself.