Well, if you are playing against a club that helped you develop, gave you support and you established a bond with, then it's pretty inappropriate to celebrate (unless it's say one of those late season games where your new team needs to win in order to win the league/avoid relegation/etc, and your former club isn't fighting for anything).
If someone like Altintop or Sahin scored against Real Madrid and then celebrated, I wouldn't care at all. If it was someone like Higuaín or Di María then it would hurt much more.
Also, it depends on where the game is played. I find that it's much more acceptable to celebrate goals against former clubs if you are playing in your new ground, and not in your former club's stadium.
It depends on the circumstances, but that's my opinion in general.
This post was edited by zarkadon on Sep 22 2014 05:01am