I'm pretty sure that the important part is gameplay, correct? In that aspect it is rather spot on. Of course, the graphics make it a bit awkward, but as I said, I think the primary reason that it is recommended that you make that clone is to get a feel for the physics of it, and in that retrospect, it was definitely a good job.

I actually like the simple graphics. That's just me though. And yeah, the game seems quite smooth and not buggy =D The fact that the next piece is shown is a bonus, and that it looks 3D even moreso =P

Now for the suggestions:

* Make the HUD show the number of lines.
* Don't use Up to rotate the pieces. I know many Tetris clones did this, but the truth is that it becomes tiresome to the hands, so moving it to a separate button seems like a good choice.
* Start improving in the score system. You may want to make it so that putting a piece gives a few of score, and making lines adds up a lot more. Every Tetris game has its own scoring system, so just make yours. You may want to give more score to making multiple lines at the same time and dropping pieces instead of letting them do the free fall.