Coming from the PC world to the Mac world, all I have to draw on are my PC upgrade concerns that may or may not apply. But, here are my concerns:
1. Major vs minor releases. Major new release upgrades seems to run heavier (slower & doesn't feel as tight) than the major new release. Although, small incremental upgrades don't have any noticeable difference in performance and do fix some bugs.
2. Recovery. When restoring after a hard disk crash on a PC you have to reinstall the OS from the DVD or CD to the lastest full version then layer all of the incremental upgrades one by one to restore the OS. This is tedious and sometimes difficult because if you don't put the layers down in the right order the OS isn't fully functional.
3. Psychological ease, having Lion cleanly installed from the factory on my new system puts me at ease knowing that I won't be lugging around extra unused "dead code" and obsolete features eating up hard disk space.
4. Training. I just want to learn one version (10.7) of the OS.
5. Patching. I'm sure Lion will have it's issues when it first arrives and I'll probably have to install multiple bug fixes early on, so I just want to keep track of one version of OS and have to worry about past compatibility issues with the HW, OS, nor applications.