State of the art or flavor of the month? For instance, the features from functional programming that C++ and Java recently (in the last decade) added weren't anything new. When functional programming started to become more popular was when their features started showing up in C++ and Java.
If people are concerned that your language is already to large than adding elements from other programming paradigms because they're suddenly what's hot doesn't seem like a great idea. It feels like some languages are chasing the crowd, which can lead to a messy language ("OOP is all the rage now? Our language is all about OOP! Oh, functional is all the rage now? Well, we just nailed on some functional features!").