If it were possible to "cause major problems with the core bitcoin features" just by making valid bitcoin transactions, then it would be very easy to attack the bitcoin network.  Think about it.  

The colored coins protocol allows one to issue custom digital assets on the most secure blockchain and trade them for other colored coins or for bitcoin, even supporting trustless coinjoin transactions.  This is big.  

The other foo-coins like NXT that implement features similar to colored coins are at a strong disadvantage:

   1.  Their blockchains are not as robust as bitcoin's.

   2.  It is not possible to trade those assets against bitcoin in a trustless manner (limiting your trading options to people you know or using 3rd party exchanges).