Well I just spent 35 hours or so playing Terraria over the last 4 days, so I liked it quite a bit. But I think I'm more or less done with it now, until I get enough bros for a multiplayer game anyway. (edit: I mean IRL bros)
So, Terraria is quite a lot like Minecraft, as you've probably heard; it's got mining and crafting. The main difference is that it's 2D and looks more like a Super Nintendo game.
The best thing about Terraria is the reward scheme. Every time you get a new piece of equipment, it helps you defeat new enemies, mine new materials and explore new areas, which then helps you get even more equipment. There's never a moment when you don't know what to do next and feel you should take a break; it just keeps you playing and upgrading your character. This isn't done in some sort of grindy fashion either, the gameplay actually changes quite a lot as you get new abilities.
I felt that Minecraft's biggest problem was that it didn't really have any well defined goals, well, besides beating the Ender Dragon. But in Terraria there's quite a few bosses, you've got a town to build, and your equipment never breaks over time.
Equipment includes swords, axes, hammers, boomerangs, bows, bombs, hookshots, rocket boots, GPS, guns, harpoons, grenades, shurikens, spears, a pile of magic spells, and bunch of other stuff. Getting new equips is probably even more fun than it is in Zelda.
The bosses are terrifying, especially since you'll probably be too weak to defeat them the first time you bump into them. Beating bosses takes planning; you need to collect a bunch of potions for buff effects, decide what weapons to use, and modify the terrain to make things easier for yourself. There's a lot of different strategies you can go with.
Another thing I feel the game does pretty well is atmosphere; it's dark and gloomy in caves, and you have to light them up as you go along. Enemies make creepy noises so you know they're around. There's all sorts of glowing items to be found in the dark. The narration can really panic you at times. The music changes appropriately between day and night, at different depths, and during boss fights.
Anyway, there were still things I disliked about the game, the main one being that there's not much of a tutorial. A lot of things you can figure out on your own, but for crafting recipes you'll pretty much have to check a wiki every 5 minutes. I was also getting really frustrated at one point because NPCs stopped spawning. It turns out I was using a bad type of block to build stuff, and they didn't like it, though the game never told me that. I also encountered a bug where one of the bosses wouldn't spawn until I reopened the game.
I'm also not a fan of the end-game content; the hard mode. It feels very unbalanced compared to the first half of the game. Enemies and bosses seem way too hard, and there's not much to do besides hunting down rare ores. I would have liked if they just mixed the hard mode content with the rest of the game, and made it a bit easier.
Overall, it does start to quickly get boring once you get to hard mode and run out of things to do, but the first 20 hours are amazing.
tl;dr: like Minecraft, but in 2D and more rewarding to play.