Atmosphere, a key word when people talk about the Metroid series, but the game offer so much more than that.
My first experience with the Metroid series was my favourite game Super Metroid and what a great experience it was. It had the aforementioned atmosphere, great graphics and sound and the most important thing, great game play. The really great thing about the game was the fact that anyone could pick up and play it, not really a gamer and just want to run through it? Fair enough. An experience gamer who wants to hunt down everything? Go for it. The backtracking to collect items to upgrade your missiles and suit was absolutely brilliant and instead of feeling boring and repetitive really added depth to the game and the bosses, the bosses were just great and each one was so different to the other.
But enough about Super Metroid, although it did get me into the series one game doesn't make a franchise and luckily the rest of the games that I have managed to sit down and finish have been just as enjoyable. The two GBA games Zero Mission and Fusion are brilliant additions to the series and are held in high esteem by fans and non-fans alike. If you can find copies at cheap/reasonable prices they are definitely worth buying. One really good thing about them is that though the basic formula is the same as Super Metroid the games are quite different and a lot easier in difficulty which is good for when you just want to play the handheld for short bursts.
Like the Zelda series the move into 3D added whole new layers to the Metroid games with the Prime trilogy and Other M adding new depths to the games with scanning your environment and a new control scheme to allow swapping between 2D and 3D. Other M even added speech for Samus and a background to the hardened Bounty Hunter.
Like the Zelda series the main reason that I love the Metroid series so much is because I just want to go back and play the games over and over, that and the atmosphere.