The legendary heroes expansion came as a surprise to me, and I jumped right into the beta without knowing anything in advance. For those who were waiting for some feedback from players before trying it out: this is what I found so far after finishing my first game.
The weapons system is far better than than before! Gone are silly design decisions such as the difference between +2% piercing or +2% blunt damage that didn't really do anything for me in Fallen Enchantress. Instead the weapons themselves now feel completely different from eachother and grant different abilities based on what they are. Now I pick weapons with a combat role in mind, as it should be. I don't like some of the weapon abilities, such as the ability to bash fully armored units away with a shield (I'll probably end up modding that out), but most are very well implemented! You can now form actual lines of pikemen who can impale multiple enemies and attack from behind friendly units. The AI isn't always using these abilities as well as they could, but overall this makes tactical combat a lot more interesting!
Speaking of tactical combat... there is now a "swarm" mechanic that make flanking maneuvers into the devastating tactics that they should be! They also make combat between armies look more convincing than before. The attacking unit as well as any other friendly unit that are next to an enemy target that is under attack contribute to the total damage. The downside to this is that I trust my heroes even less than before to hold the line as they get overwhelmed easily. I'm not sure if I like that or not. On the one hand it feels more realistic, but my "legendary heroes" feel a bit underpowered. Especially at lower levels.
As for the heroes, the new hero system feels like one of the biggest improvements. Heroes are attracted to successful kingdoms, which makes a lot of sense to me. The attraction is measured in the new "fame" resource. So no longer are they exclusively found at random in the wilderness (I used to end up with rediculous amounts of heroes in my Kraxis games in FE). There are still quests that award heroes so that not all the sense of adventure is taken out of hero recruitment. Big improvement! A letdown is that to my horror I found duplicate heroes! I really hope that this is a bug that is going to be quashed during the beta. All the factions of the same alignment seemed to have the same selection of heroes. Not even the names were changed.
There are also many new tactical battle maps! There is one that I despise, where my units are placed on a mountain peak at the start of the combat, but the rest are very well made! Siege battles no longer take place in front of the city, but in the middle of it. I don't really like this change because currently my units walk straight through statues and other obstacles. Aesthetically I don't like it either, because the civilians look like they are going about their daily business, shopping at the market in the middle of a raging battle. I'd prefer the old sieges back, but failing that, to at least have the civilians removed from the building tiles! (there also seems to be a scaling error with the kingdom civilians being much taller than the troops).
I've experienced a few weird glitches that I never had in FE, such as the lines in the tech tree getting messed up or techs losing their labels and multiple guardian statues spawning in sieges. I'll file more detailed bug reports when I get around to it, but I had this happen several times after multiple restarts of the game.
My biggest disappointment so far is that the AI still has the exact same problems with clearing monsters and looting their lairs as before in FE. It really hurts my sense of immersion when I progress through the story, rescuing the scattered civilians from their hideouts and refugee camps at the start of the game, building up my clean and orderly kingdom from the ashes of the cataclysm, only to find that the AI kingdoms and empires are still utter post-cataclysmic hellholes with refugee camps, bandits and monster lairs still littering their lands. The AI seems to be no civilizing influence to the world at all! The monster lairs will invariably have been allowed to spawn huge hordes of roaming creatures which I will have to hack through to even move between the AI cities. Rarely I witness them attack the AI, but mostly the AI player units and monsters seem to coexist just fine. Even unescorted pioneers scurry in between giant spiders and hungry lizards without worries. Even when the AI does kill monsters they leave the loot for me to pick up. I'd rather have monster lairs and "free loot" for the human player disappear automatically inside an AI's borderd after it has settled an area for a long period of time rather than how it is now.
Overall I'm pleased, and I recommend getting the expansion to any fan of the game. It is still very early in the beta, but there is room for improvement!