1) It's very similar to 3.5e D&D as you've had lobbed at you by people in this thread, making it fairly easy to pick up.
A lot of the concepts are easily carried over. The way attack rolls and saving throws are handled is pretty similar, except there are 6 saves instead of 3. The races got broken down into Heritage, Culture, and Background to avoid reflections of real world racism being carried into the fantasy realm. Only your background determines ability score bonuses because it's what you've done and how you live that decides if you're strong or smart or something similar.
Aside from renaming three classes all your favorites are there. The monk became the adept, the barbarian the berserker. Both to reduce the racist connotations of the terms, the former relating to orientalism while the latter is a catch-all term for 'uncultured outsiders'. And then Paladin became Herald just to get it away from the Crusades.
But you'll also get the -excellent- Marshal for a new type of fighter character who throws out commands and buffs to their allies not through bardic music-magic or prayer, but coordination and training! "Rub some dirt in it and walk it off, soldier!"
And then there's Combat Maneuvers. You kind of got some of them introduced in the Tome of Battle and the Book of Nine Swords... A5e gives them to -every- martial character in the form of combat traditions to add another layer of character customization.
2) The changes are almost invariably for the better.
Advantage and disadvantage were honestly the biggest and best rules change that 5e had. I mean that, sincerely. By adding in a simple way to resolve situations where you gain 'a benefit' or 'a penalty' it really cut down on the Pun-Pun bonus stacking nonsense where you needed to look through 20 splat books to maximize all the +1s you could get to any given roll.
But it -did- lead to some lack of granularity. In comes Expertise. Add a d4 to the roll. If you get expertise from multiple sources, increase the die size one step for each up to a d10 maximum. Retains the simplicity while adding some granularity.
Similarly, the spells got reigned in a bit. It's not -perfect- but a fireball starts at 6d6 instead of 8d6, while lightning bolt -is- 8d6. Which makes consideration of positioning a lot more important in which spell to cast instead of just "Fireball. Yeah, I know the room is only 20ft wide. What do you mean friendly fire? Sorry, guys. It does the most damage."
3) All the pillars of gameplay are given consideration for every character.
In 3e if you wanted someone to talk to the king your barbarian and fighter, ranger and druid, monk and wizard, all shut their collective yaps and dithered with dice-towers while the Bard, Sorcerer, or Con-Man Rogue did the talking so they could roll the big Diplomacy check. 5e carried that forward to an obnoxious degree. Similarly, the Ranger/Druid/Rogue handled all the exploration mechanics like finding trails or shelter or food... or in the Rogue's case: Traps and Treasure.
Once more the rest of the party twiddled their thumbs while one or two characters stood in the spotlight for a specific task... but -everyone- shares the spotlight in combat, since everyone's usefulness in combat needed to be 'balanced'.
A5e changed that. Now your Fighter gets built in benefits for social encounters. And a list of exploration options they get to pick from for when they're not in a fight. Same thing for your Wizard and Berserker and Adept. Everyone gets benefits they can bring together as a -group- in any situation.
4) Prestige and Journeys make things even deeper. And Destiny gives you somewhere to go.
Prestige is based on your level and what you've accomplished and it has implications toward your fame and social standing. Meanwhile the Journey gameifies travel by letting players choose roles and tasks to do on the road that actually have rules support for them. There's even options for stuff that is really player specific. Like the player who likes being there for others and offering support on the road being able to take up foraging for spell components or straight up -cooking-.
Or the edgelord rogue with the dead parents avenging his dead -wolf- parents getting to mug travelers along your path. (Brennan Lee Mulligan reference)
Meanwhile your character's Destiny is chosen at level 1 and provides you with small goals to achieve as you gain levels. A clear narrative for your character... but. You can also -change- your Destiny, which is awesome and incredibly thematic for High Fantasy.
5) Monsters are WAY more balanced.
CR is a lot easier to figure out and deal with. Point-based design systems were used behind the scenes to give monsters a 'point total' (give or take) to determine their challenge rating based on their damage output, hit points, armor class, etc. It's kind of complex but once you understand it, it's super easy to make new monsters.
Or you can use the monster-builder tool on the A5e.tools website... because...
6) MASSIVE WEBSITE SUPPORT!
Not only is the world's largest SRD available for A5e, giving you access to -so- much material out of the gate even if you never buy a book, but the tools site provides you with excellent tools created by Russ to help you build and design. It's -so- good. And if that's not enough? These forums exist. On this website. Where you can talk to not only the EN Publishing team, but also pretty much every Third Party Publisher because they don't silence us, here.
Heck. Sometimes they encourage us to reach out to the community because EN Publishing recognizes that third party support grows the community, even if it doesn't expand the core system/SRD.
7) The problems it has are problems 5e has.
Dexterity is kind of broken, if we're honest. With finesse weapons allowing you to use dexterity to AC, a strong saving throw, lots of skill checks, initiative, attack rolls, and damage rolls? It truly is the S-Tier character attribute in a league of it's own. And 5e has the exact same problem, so you can't really "Escape" the issue by using 5e instead of A5e.
That said... A5e does reduce the problem slightly. Initiative defaults to your Perception skill instead of being a straight Dex check, but it can use any attribute and any skill, depending on the situation.
For example you're in a Mexican Standoff with the bad guy and the rival adventurer. D&D 5e would have you just roll dexterity to see who gets to swing first, but you could argue in A5e that -Insight- would be a better roll. After all, you're trying to read your opponents and act when they're either about to make their move or have let their guard down. Or a Deception check to convince them your guard is still up or that it's dropped (depending on how you want to roleplay the initiative check).
Similarly, in different situations you can use different attributes for skills. Strength Intimidation checks are a great example.
So even with the same problem 5e has, at least A5e reduces it -marginally-.
8) First and Third Party are blended really well.
Because most of the Third Party Publishers working on A5e are also freelancers, we get to work for the Gate Pass Gazette and other official A5e material. And because EN Publishing values it's third party friends they even allow us to republish our work for EN Publishing a year later in our own titles. The book I'm working on, right now, is Martial Artistry. And in it, I'll be including two combat traditions that appeared in the Gate Pass Gazette over the past two years.
What other company do you know of that would allow it's writers, designers, and creators to retain creative control over their own material after only 12 months of exclusivity? The sheer -generosity- of it is mindblowing to me. Add to that the support in connecting third party publishers together through discord servers and the like has been huge for networking and interaction.
Putting a bunch of creative minds into one place and encouraging their creativity? Gets you a -lot- of wild and interesting ideas. Maybe not all of them are great, but they'll smash into each other until something good comes out of it, nine times out of ten.
9) Gate Pass Gazette means there's always fresh content and new writers
If you haven't seen it, yet, the Gate Pass Gazette is a monthly publication EN Publishing puts out where they get freelancers to submit pitches for articles, pick the best ones, and have them write. There's 4 articles of 2,000 words. 8,000 more words of more A5e content every month. The patreon is fairly cheap, and while you may not like -everything- in a given issue, you're very likely to find at least -something- you can use.
Plus every issue gets compiled into a hardcover book every year, so you're guaranteed a 96,000 word book of content every single year, on top of the Dungeon Delver's Guide, the Voidrunner's Codex, or any other materials EN Publishing puts out in a given year.
TLDR
A5e has everything 5e has going for it except the name D&D, adds more to it, and flatly does a bunch of it -better- while encouraging and supporting a strong third party publishing community to keep providing more content, more interest, and more word of mouth about the game.
WotC could never. Not anymore, at least. They got too jealous of Paizo's Pathfinder and tried to kill the goose that lays golden eggs.
