I had the same reaction when I played the free version. It was just really boring.
Some friends of mine really raved about it and I explained why I found it dull and they said the full version was much better. They were setting up a server for everyone to play on so I thought I'd give it another shot and get the paid version.
I found that once you add in the day-cycle and have zombies and skeletons coming at you the game gets a lot more interesting. Building stuff feels like much more of an achievement because you have to collect the resources for it and deal with all the horrible monsters that want to kill you.
The paid game also has a lot more content than the free version which really increases the variety of things you can make.
But yeah, you're right the free version really isn't a good advert for the paid game. If I hadn't had friends who persuaded me that it was worth giving another chance I would have given up playing it altogether.
Huh, I can't believe people are still posting about this year old post. Also, I've tried a pirated version a year ago as well to find out what the full version had. There's really no point to this game for me still as it's always just dicking around with monsters randomly spawning and boring material grinding. At least with Terraria that game had a real "adventure" type of game mode you could play with.
The one game I've still always liked the most of the "sandbox" genre would be Garry's Mod. No ******** time wasting with finding\mixing materials, just spawn explosives, NPCs and contraptions that are really more marvelous than the 2D clip art that people are proud of for some reason. Hey guys, you could do the same thing with Microsoft Paint.