Check out the articles in my signature for how I do my components. They're basically just arrays of components (one per type) and another array of component masks. An entity is the aggregation of all the elements at a given index in the arrays, where the component mask describes which components "exist" (are turned on). It's cache friendly and requires no memory allocation past the initial allocation of the arrays.
I'm not sure I agree that components must be data-only and you also specified that systems operate only on groups of components when they all exist. Not sure what you meant by that but doesn't that negate the plug-and-play nature of ECS?
Nope, my system is perfectly plug-and-play. The systems read the component mask to determine if they should act upon the component data for that entity. If you're "adding" or "removing" a component, you're just modifying the component mask to change the visibility of the component to the systems.