Don't judge a book by it's cover.
Terraria is a bit hard to describe - your experience will truly change as you play the game. Upon entering the world, many will find it similar to Minecraft, but in two dimensions. This is not necessarily a bad thing - the two dimensions can make for quite a bit of nostalgia and I personally find it much more fun and easy to navigate than Minecraft's vast plains. Despite the two dimensional feature, it sports a clean, neat graphics style that reminds me of something you would find on a 16-bit system. But, if one sticks with the game, it will become more of a "Minecraft meets Metroid meets RPG". I rarely felt bored early on in the game, always tasked with something to do or to discover - Gather more iron for a new pickaxe. Build a new story in my home to welcome the Merchant into my living space. Stock up on shuriken and confront the massive boss, Eye Of Cthulhu. The beginning of the game is a blast once you get the ride going.
"So what?" you think. "Metroidvanias and minecraft-style games have been done before." True, but was the Mona Lisa judged by the individual paints used to make it? Terraria is much more than the sum of its parts. Discovery is key to maximum enjoyment. On my first day, I was thrown into the world with literally no knowledge of the game (A friend had recommended it). Ten in game days later, I was creating my first set of armor out of refined iron. Jump forward another ten days, and I'm flying circles around the Eye Of Cthulhu, throwing shuriken like mad while I use my epic Rocket Boots to evade his attacks.
However, things get a bit slow near the endgame, considering there is no "goal". You truly have to make your own fun. But of course, the game unlocks new things bit by bit for you. Defeat the massive Skeletron and unlock the deadly underground Dungeon. Explore the depths of that and discover a passage to the Underworld. Accidentally summon the horrifying Wall Of Flesh and barely make it out, unlocking Hard Mode.
Everything I've described so far is only a tiny portion of Terraria. It's a massive game that I wouldn't be able to describe with this sites 400 word limit. Since I couldn't fully cover every subject, here's some scores.
Fun : 8.5/10 (Short lulls of boring on occasion)
Sound : 8/10 (Nice sounding tracks that can get a bit repetitive)
Graphics : 8/10 (Nice, clean look)
Worth your money? : YES