In response to:
Everything that jmolly says is sincere, and rings true enough, but decoupling data and functional behavior is only part of it. Indeed, OOP was designed specifically to couple data and behavior together, so some people (at least) think that this is a good thing.
In my mind, the difficulty lies not with OOP per se, but in the thought-patterns and practices that have grown up around modern OOP programming, and the mental confusion that we have between the construction of models, the design of algorithms, and the creation of machines to help solve a particular problem. To a large extent, we lack the intellectual tools required to understand the problem; or, at least, the use and understanding of those tools is not widespread.
Personally, I suspect that there is a significant difference between product-centric thinking (design the product, ignore all else), and product-line-centric (or capability-centric) thinking (design the machinery that will make it trivially easy to design the product). This, for me, is a more important distinction than OOP vs FP, or any other dichotomy that you might care to mention.
So in focussing our OOP modeling efforts on the product itself, it becomes easy to fail to model potential variations in the product. To compensate, we attempt to use "best practices" and "design patterns" to ensure that we produce software that is flexible and malleable, but because we do so without any concrete reference point, we end up lost, making software that is too abstract and too complex.
We need to take a step back, and realize that software development is not abstract. It is concrete and is embedded in a social, political and business environment. It only appears to be abstract because there is so little communication between the software itself and the other stakeholders in the environment.
We do not, rightly, start our engineering effort by writing code. We start it by engineering the social conditions and the organizational structures first.
Once this is done, writing the software becomes, perhaps more straightforwards, once the goals and objectives of the organization are well aligned with the goals and objectives of the development effort.
See also Conway's Law.
I am not suggesting that I am right here, mind you: this is just the way that I feel; my gut instinct ... so perhaps some Socratic questioning might be in order here:
Let me start.
What problem, as software developers, are we actually trying to solve?