The price tag 
Especially because the 3DS is due out sometime soon, and I don't know how much, if at all, the DSi will go down.
That's the same reason I havent upgraded either. Once I spent the money on the DS I figured I didnt need to spend it again for a few features I probably wouldnt use. I feel mostly the same way about the 3DS too, but I dont really use my DS for anything but Pokemon and a very small list of other games.
I was sorely tempted by HeartGold/SoulSilver, but I think probably quite wisely I kept the cash in my wallet that time.
I absolutely love HeartGold, but Gold was always my favorite game so it was nice reliving the memories with all the new features tossed in. If you're going right for Black/White then I'd say skip HG/SS, but if you are just looking for another Pokemon game to play and you dont want to jump back a generation I'd say it well worth the money.
