How far I should go with customization ........

Almost every SharePoint project starts with elaborating  what the current version of SharePoint supports as Out of Box  feature .

But at the end of the day , level of customization we require in these OOB feature is huge.

To make it easy for future upgrades and better performance and really really get what Microsoft promise , we should be inclined towards no  customization.

What's the difference between Abstract classes and Interfaces?

Features                                      Interfaces                                abstract class

Multiple inheritance	A class may inherit several interfaces.	A class may inherit only one abstract class.
Default implementation	An interface cannot provide any code, just the signature.	An abstract class can provide complete, default code and/or just the details that have to be overridden.
Access Modfiers	An interface cannot have access modifiers for the subs, functions, properties etc everything is assumed as public	An abstract class can contain access modifiers for the subs, functions, properties
Core VS Peripheral	Interfaces are used to define the peripheral abilities of a class. In other words both Human and Vehicle can inherit from a IMovable interface.	An abstract class defines the core identity of a class and there it is used for objects of the same type.
Homogeneity	If various implementations only share method signatures then it is better to use Interfaces.	If various implementations are of the same kind and use common behaviour or status then abstract class is better to use.
Speed	Requires more time to find the actual method in the corresponding classes.	Fast