The good: The classic civilizations all the way 'til now is fully balanced for this game, the patches keep the game from getting out of control, multiplayer is still top notch.
The bad: The AI especially the first one lacks the age of kings and the conquerors punch, you need a powerful computer to use the ultra-high-definition graphics, making your own civilization would be nice, can't play without paying Steam in full price, can't transport trebuchets anymore.
This is a no brainer since 1997 when the first game came out starting with 13 civilizations. Add The Conquerors and it's a total of 18. The HD with the DLC would be 31. And at this point in the definitive edition, a whopping 42 civilizations! The replay is really trying out all those civilizations along with all the changings and AI callings. But the AI, especially the first one needs some work. The original AI in both age of kings and the conquerors starts kicking gear when you load the game you save but does not go beyond castle age. The AI on HD and Definitive Edition is an improvement though even on the easiest settings, it's still a major challenge to deal with. Even though it's minor for now, you won't be able at this point move a trebuchet across the water via transport ship. So, you'll be forced to make castles on the other islands to make up for this issue.
The graphics looks as good as Age of Mythology, even better. If you have a powerful computer, you can use the ultra-high-definition graphics at the price of performance. But if you own a laptop, you might encounter a major slowdown to a near crawl. The sound on the other hand is great with the music redesigned for this game. Along with voices, sound effects etc.
Indeed, trying out civilizations is a lot of fun. But there's one thing you might need to do: Sign up to Steam and buy lots of Steam cash in order to buy this game along with all DLCs (including UHD for the laptop). The multiplayer is well up to date and the patches keeps the game from getting out of control and continue to keep the civilization from getting out of balance.
To keep it simple, if you want your classic game push to the limit, Age of Empires 2 Definitive Edition is the one to get. It will an interesting comparison to Age of Empires 2 HD with DLC and Age of Empires 2 The Age of Kings and Conquerors as it will give you infinite replay value. The only thing missing is simple really: maybe in the future, you can make your own civilization, even if it reaches up to 100 hundred civilizations. Enjoy this game if you can setup access to a Steam account to make it work.