In the modern secular mindfulness movement, there seems to be a growing understanding that mindfulness by itself is not enough. That's a good development, understanding that mindfulness is an important skill that works best when combined with other skills and processes.
Interesting but confusing ways are popping up to try to deal with this issue. I've heard assertions that mindfulness necessarily includes curiosity, or a positive intention, or warmth. While those can all be quite useful, they aren't mindfulness. You can be mindful of curiosity, or mindful of a positive intention, or mindful of warmth, but that doesn't mean they are the same thing.
To illustrate this, think of your tastebuds. They can be aware of tastes like sweetness, bitterness, or sourness. The tastebuds themselves aren't sweetness, they aren't bitterness, and they're not sourness. The tastebuds have the capacity to experience and identify tastes, but the tastebuds are not the tastes.
And yet the whole purpose of tastebuds is to taste. So you need tastebuds _plus_ something to taste.
The 'plus' in Mindfulness+ represents something similar: mindfulness is combined with something to be mindful about. The + sign serves as a reminder that we need to add something to bare mindfulness. And the question "plus what?" prompts active thinking about what that something extra might be.
In the context of Early Buddhism, mindfulness was practiced in conjunction with other elements such as clear understanding (sampajañña), factors of awakening (bojjhaṅgā, of which mindfulness is one), or other aspects of the Eightfold Path like Right Effort, Right View, and Right Speech.
