I rely on 2 long-standing unmerged drivers on a daily basis...
yaffs2 (http://www.yaffs.net/) is an excellent NAND flash filesystem with all necessary features and acceptable mount times for large devices. It is widely used in embedded systems shipping today. A barrier to merging may be the existence of a compatibility layer allowing the filesystem to compile on other platforms and bare metal (eg: RTOS's & bootloaders).
drbd (http://www.drbd.org/) is a distributed block device driver used in high-availability systems and an integral part of the Linux-HA architecture. A barrier to merging may be existing features in the main line with partially overlapping goals.
I've asked on the mailing lists of both these projects when they will be merged quite some time ago and got vague promises, but no visible effort towards inclusion. They are both backed by small companies which may be unwilling to give up control. They may simply have other priorities.
I'd like to see a way for these sorts of projects to get merged. Perhaps some body could be persuaded to provide the authors with a financial incentive to get the merge work done.
Both of the examples above are large bodies of work which I'm very glad exist in any state of merging. Given the effort and generosity of their authors over a long period, I wouldn't like to see the projects forked for inclusion.