Terraria's basically a "metroidvania" plus building. Don't let anybody tell you it's "2D Minecraft", because it isn't. Adventuring and exploring is primary, with combat as a secondary focus, and building and sandbox-y elements come in after those two (though very closely). Not to say that you can't build, of course.
I've never played Castlevania and I've never played Metroid outside of that one for the GBA, which I tried out at a Walmart when I was a kid. I do however like adventuring and exploring. I also like combat and I like building things. Thing about sandbox elements though is that sometimes they leave me with some degree of choice paralysis and other times if I stick with it for long enough I get bored and need to take a hiatus. Not really a problem but it might result in me taking breaks for one to three months at a time.
Also the first thing I thought of when I saw this game was that it did remind me of Minecraft, which I like very much despite my only experiences with said game being the single-player version. I don't think it's a bad thing as I also thought to myself that Terraria reminded me of the fun aspects of Dwarf Fortress and Haven and Hearth, which would be the building and the exploration.
Either way I think I'm sold on at least playing this game in between bouts of Fable: The Lost Chapters and Moonlight Online. Just got to devote $9.99 of next week's $50 check to it, so if I'm able to do that the earliest I might be in Terraria would be somewhere around the 15th to the 17th. I look forward to playing with you guys.