I'm a bit sceptical about Terraria... I mean there is high battling factor low building factor. Minecraft is reverse. And dwarf fortress is just perfect balance of both and unlimited way more complex (yeah i could go on...). As for mod-ability Terraria creators are not stupid- why let others create infinite world and items and fun when you can create DLC and get paid for them. Notch wanted to do the same but it wont work out as well because everybody is already modding.
So yeah shame that its not editable in any way and not a lot of stuff can be built (i mean not house, house, house with chandelier, house... but something more complex and interesting...)
Since all NPC types are linked to specific rooms, the building is required if you want to proggress. Also it's just alpha, there is a high chance that they will include NPCs which require more than just a room with a table and a chair, maybe even different room types.
And to those who asked: it is possible to remove the background "ground" wall and build underground.
Even in the leaked alpha the netcode is not so bad. It sends a huge load of traffic so there ought to be teleporting monsters, desync and extremely laggy chat, so high bandwith dedicated servers are required to play with many people.
However I am sure that they will fix it (if not already), because the leak was a very early alpha, and now in LP they seem not to have such noticeable sync problem.
Also the monsters are processed client-side, so it may lead to "teleporting" monsters or an already dead monster killing another player if the lag is extreme, but the fights are always smooth. It's a very important factor in a platformer.
I will definitely buy this game. It may become a bit tedious with time, but it's a great fun, a lot of nice music and style. It's noticeable how developers put their heart into the game.
P.S. PM for multiplayer