As a boy in 1999 who grew up playing Nintendo and Playstation 1, heading into Sonic Adventure was simply mindblowing. Even today the graphics hold up, the gameplay holds up and it's got enough variety in there to keep you entertained for hours. It's unique gameplay is held together by the use of open world elements which can be explored in a unique way with each of the characters' abilities, namely Sonic, Tails and Knuckles.
Just the feel of the game in your hands is far beyond what many modern games have accomplished, the variety in the running speeds controlled by the analog stick beats out games such as Grand Theft Auto V and rival that of first person shooters, which makes the gameplay of Sonic Adventure incredibly advanced for it's time.
The gameplay itself does not hold the player's hand, instead providing clues to getting through certain situations but supporting the player's abilities enough to ensure a fast pace through each and every level (excluding the character "Big" The Cat's levels)
The size of the levels is breathtaking even compared to games of today, some of these levels are tenfold that of the open world areas and truly give the illusion that Sonic is travelling miles upon miles. If you want a perfect run on a level, be prepared to retry a few times as it takes skill and control. The sheer effort that the development team put in is beautiful to experience, and puts to shame many development teams of today - SA constantly switches up the gameplay with FUN mini games to keep even the busiest of minds engaged and surprised. Some levels seemingly morph into totally different dimensions and this truly makes the game a trippy experience for those who are green-inclined.
Another important highlight is how friendly and even educational the game is for kids, the developers include important messages on childhood by adding the Chao - a small creature which hatches out of an egg and can be sent out to race other Chao - this is completely optional to do.
The Chao garden/race gameplay shows players that raising their Chao is not easy, if they throw eggs around.. The Chao will be affected, if they don't keep an eye on and provide for their Chao.. The Chao will also change accordingly. It's a beautiful message about how parents should behave, which i'm sure has been done before but messages like this are all but missing in the more violent video games of today.
Sonic Adventure to me is a swan song for many gameplay mechanics created through the nineties, and the Sonic levels in particular are a true 3D representation of the 2D era sonic games, the envelope is completely pushed when taking into consideration what the Dreamcast's limits were.
Not only is this game a technical masterpiece for it's time and even years after, but it's purely fun experience which cuts out the fat and throws you straight into the action.