First of all, you should increase the speed of the blocks falling over time. Urgency is always a great thing in puzzlegames, as it prevents the game from becoming stale.

What I've noticed is that... well, it seems as if it is impossible to lose, really. Either you want to lose or you won't, as you can keep placing the pipes so that a line is complete, which will remove that line and you can go on. That, my friend, isn't so good. Let's try to make this different: Pipes disappear when you connect four or more of them together. That does also help to set your game apart from tetris. Combine that with my neat "water-comes-down-the-pipes"-idea and you have even more urgency which is, as I've stated, the key to true awesomeness in puzzlers.

Special missions might also help, for example, the player may earn a nice bonus when he manages to connect four "+"-parts in the next thirty seconds - otherwise he'll lose points, gain a few useless pipes, get only bombs or whatever. Surprises are always good (almost as good as urgency )