Every OSR is defined by two criteria:
The choice of inspiration OS (IOS) which serves as the inspiration to the OSR.
The degree of adherence to the design and implementation of the IOS.
Obviously, these re-creation efforts do not seek to create a direct facsimile of the original OS. The reasons for this are two-fold:
Firstly, any OS which was simply a copy of another OS and that did not vary in any way from the original, would be, by definition, redundant. By way of example, let us say that a group of people might set out to create an open source clone of the Microsoft Windows operating system so that they could release it as freeware. In this example, it is the licence and the development model which vary from the original source OS.
This first point is dependent on the motivation of the developers - why do they want to create this new OS? Example motivations and ideological aspirations for such a project might include, in varying amounts, factors such as:
The re-creation of the best parts of an older operating system, but utilising more modern methods of design and software engineering.
The re-implementation of another OS, under a more open development and distribution model.
Secondly, the IOS system must, logically, predate the creation of the OSR; it is therefore natural that some advancement in the field of operating system development and hardware development would have occurred in the time between the release of the the IOS and the start of the re-creation project.
Omission and Addition
The developers who are involved in the actual creation of the new OS will almost certainly have some opinions on the subject of OS design. The two ways in which a thing which is inspired by something else can deviate from the original are by omission and by addition. The creators of a clone OS would have less motive to recreate a feature which they regard as unimportant than they would have motive to create a feature which they regard as relevant. This is not to say that the omission of a feature is evidence that the re-creators felt that it was a mistake in the original OS. An omitted feature may represent a function whose importance has become depreciated by time. For example, an OS created in 1994 might feature a very well designed and comprehensive Internet modem dialler; however, the importance of this feature is depreciated in 2006. An OSR of this sort might place only limited importance on full and comprehensive floppy disk support, for the same reasons.
Also, an omission might represent a compromise on behalf of the developers that was brought on by practical considerations. If a team were to create an a new OS which was similar in function and design to MacOSX, they may decide to stop short of recreating all of the iApps which are bundled within a MacOSX distribution. Having the complete iBundle set of applications might be desirable in a MacOSX clone but its omission could be the result of the practical resource limitations which face any software engineering project.