Terraria combines the best of side-scrolling platformers (think Metroid) with the variety of RPGs (think Final Fantasy 3) and throws it all into a completely consumable/destructible/rebuildable world (think Minecraft).
After an hour of play I am very impressed with the game. I was worried initially that a keyboard and mouse would not fit well for a sidescroller, but I was pleasantly surprised to find the controls to be responsive and easy to learn. Mastering some of the more complex movement will take some time which gives the player a rewarding feeling when they complete a relatively complex jump.
For those players that suck at timing jumps and landing on near invisible parcels of dirt, there is the option to dig up and pile a bunch of dirt (or clay, wood, stone, sand, or any number of harvested blocks) to get to the desired point. This is taken even further when players start building houses and other dwellings out of the available materials and start filling them with crafted items.
Crafting within Terraria is insane. There are TONS of items to make that unlock other items that unlock other items that; OK you get my point. There is everything from flower pots to grappling hooks. Different tools can be crafted that perform different tasks. Weapons and armor can be crafted and there are super-secret hidden versions of each that really drive the player forward to keep exploring and trying to combine/find new ingredients.
The crafting is recipe based, but the recipes are not something that are known. If at the applicable crafting station (work table, anvil, furnace, etc) with the correct items, the end product will show up in the crafting menu. For example, a wooden sword may require 3 pieces of wood (I forget off the top of my head). The player can go along finding out the combinations themselves, or any number of available websites can spoil the fun for them. In some of the rarer item cases, spoiling is about the only way to go.
Those rarer items will be needed, because come nightfall, just like in Minecraft, the monsters come out to play. However unlike Minecraft, Terraria is dead serious about its combat. The monsters that appear during the first night of the game are no pushovers: flying eyeballs, zombies, and the ever-annoying slimes (which are also present during the day). Kicking it up a notch, there are mega bosses like the Eater of Worlds who take up the better part of a screen and appear after certain events like smashing three shadow orbs occur.
Most amazingly, we've only scratched the surface of Terraria because there are other things like NPCs that will inhabit your buildings and sell you goods or provide services (think RPG NPCs like shop keepers and medics). Plus we need to keep in mind that the world is randomly generated and includes an entire underworld and possibly even sky castles. And again, the entire world is destructible down to the very last block. A player can craft a few dozen bombs and blow their way to hell!
Oh and did I mention it has multi-player which allows several friends to play together in the same world and enjoy the same batshit craziness together? Oh, I didn't? Well there's that too. Terraria is awesome.