1. A Plain Old CLR Object that does not derive from a base class, or implement an interface, which is defined as an integration point for a framework.
Infrastructure frameworks often brag these days about being POCO-compatible. This is in some ways a victory, because it used to be quite the opposite. In order to use an ORM, or an MVC framework, or really anything involving a "model", you needed to inherit some framework base class before it would recognize and respect the role of these objects in your application. But we're past that now. Today you can use POCOs even with frameworks that come directly from Microsoft, like the Entity Framework.
"Convention over configuration" is a similar principle of framework and infrastructure design that has been lauded for a few years now, and is finally hitting more mainstream shops. It trades on the ability to reduce the number of design decision points, allowing you to write less code, and do it faster. In other words, why take all the time and effort to deal with a complicated configuration mechanism capable of expressing myriad options, and then a complicated engine capable of processing all those options, when any one project is only going to use a slice of it all. With convention-based frameworks, just follow some simple rules, and let the framework handle all the details.
For a long time, I was really happy about this. But today I realized that for all the benefits of simplification and boilerplate elimination, there are some pretty serious tradeoffs being made. Yes, the benefits can be absolutely huge. The gains in consistency, speed, reliability, and flexibility can't be overstated. It saves you from thinking and worrying about a lot of things that are fairly arbitrary in the context of the larger business problem.
Yet for all my appreciation, some shine is starting to come off this penny for one big reason I like to call "context-smearing". Both POCOs and conventions gain their structural homogeneity and rapid development by diluting object responsibilities, and trading away discoverability and expressive design. They take contextual info that ought to be localized, and smear it out between layers, across multiple or even many different locations in code. I am becoming increasingly careful about making this trade-off. I have seen the price it exacts, and I think there are definitely cases where that cost is probably not worth the gain.
So the big problem with both POCOs and conventions is a loss of context. What's so bad about that? In a word: coupling. Regardless of the absence of tangible framework attachments like base classes and interfaces, your POCOs and conventional objects are still tightly coupled to the framework. But instead of looking at the object and seeing the coupling, it is made almost completely invisible.
When you write a POCO that a framework needs to process, it needs to be simple. Anything too complicated and the framework won't understand it or will make an assumption that doesn't hold, and then the whole thing comes crashing down. Under this pressure your POCOs get dumber and dumber. Unless you're very diligent, before long they are not much more than simple property bags. And a bag of properties is no model. It's just a bunch of data.
With the behavior driven out, there's no context with which to understand the meaning of the data, the relationships, and the references. There's no indication why this property is reproduced over there, or why that container seems to exist solely to hold a single other object. Instead, this knowledge is encoded into all the places where the objects are used. It is distributed and diffused throughout the application, so that in no one place--or even handful of places--can you look to see the true meaning of what that object's role is.
The consequence of this is that points of consumption become fountains of boilerplate and redundancy in your code. At least it's consistent, though. So naturally you'll turn to conventions in order to scrub away some of this homogeneous noise. A good convention-based framework can look at your model, look at your usage, and determine all it needs to know to glue them together. This really streamlines things. But soon enough you've compounded the problem, because conventions actually encourage the exact same diffusion of context. Only conventions grind context down to an even finer sprinkling of dust, and then scatter it on the wind.
It should be clear enough why this is dangerous. It's very easy to slip from here into what John Cook calls "holographic code", which he describes far more elegantly than I have here.
But take the sales pitch with a grain of salt. Conventions will make some of your code simpler, but they can also easily obscure the problem and the solution--or even just the actual model--behind the patterns mandated by the mechanism. An experienced "framework X developer" can see right past that noise. But should we really need to be framework experts just to understand the business problems and solutions? Solid and inviting framework documentation can go a long way in helping people see the signal in all the noise.
Likewise, POCOs are tremendously light-weight, quick, and flexibile. This commonly comes at a cost of design confusion or opacity of purpose when seen separated from their points of consumption. Establishing context is crucial in avoiding confusion and misuse. POCOs should be small and focused, not general and reusable. And they should be located as close as possible to the consumers.
While the problem could get very bad if you're not careful, it's rarely inevitable. And the speed you gain for the 80% case can often be invaluable. You just need to be aware of the pathologies in play, and take steps to mitigate them. If you're lucky, wise decisions on the part of the framework designers can reduce the number of compromises you have to make. But even more important--because it's actually under your control--wise decisions on the part of your development team can contextualize the anemic and opaque bits, limiting their reach.
My advice is to be aware of the problem, and tackle it head-on. Whether by careful coding or clear documentation, be diligent in providing context where the frameworks will tend to suck it away.