Since I've put my fair share of tenths of hours on the Minecraft Beta (both single player survival and multiplayer survival) I've got to say the following;
My first reaction after playing it for a few hours;
"Holy shit this is the game I've been looking for my entire life!" ...Unfortunately this isn't my opinion atm. Please read on below.
At its current state the game is not really something I can enjoy playing alone so I've given up on single player because I don't feel like I do something that matters on the huge worlds that get generated. There's no "meaningful" relationships to build with NPCs and there's no character development (and differentiation-anyone can use the same exact items) or some other stuff like creating a family and staff like you can do in the
Harvest Moon and
Rune Factory series.
The game is way too sandbox-y. I don't need a goal to keep playing a game but this total lack of progress (and no, I don't think of the grind to get to full diamond gear progress, it's just tedious) is killing it for me. There's nothing to be looking forward to after you obtain the best tools and armour. You can conquer the Nether and make a base there but still there's no sense of achieving something in that. You are just building stuff to build stuff.
So I mostly bother playing with a friend online whenever we can. We pick a remote place and make our base till the server unexpectedly goes down forever or someone griefs us for good.
I've had some fun on multiplayer to be honest. Once I made a small base in a cave at the side of a cliff where there was sand everywhere outside and then slowly I started gathering dirt and covered all the sand and planted trees in my yard. It was cool how that barren place turned into a lush forest. People would come and chop my trees for wood and some wouldn't bother re-planting them but some did. It was cool. I had an HD texture pack (256) that made the game look pretty awesome. Especially the cobblestone that made everything look "medieval" let's say.
Bottom line, I think Minecraft has a LONG way to go to be called something you can play while putting your heart in doing so. Atm it's barely fun to play with a bunch of friends, but then again everything is much more fun when you play it with others. Single player is dead for me, Survival or not. I mean, endless exploration and building is fine and dandy but it gets boring real fast.
I wonder if they're going to focus on more RPG and Sim elements in the future. I'm not sure if the casuals would like more "RPG gibberish" in their game. I'd love some sort of middle ground between Minecraft and Dwarf Fortress (the first feels totally aimless, the second prohibitively intimidating to the newbie, especially if you're not an ASCII afficionado)