Linux grows up less than Mac OS, Why? It is free, secure, portable and (relatively) easy of use (Ubuntu).
The most important reasons are: Mac still offers a better user experience (as Mark Shuttleworth admits) and not enought commercial software are avalaible for Linux.
So, what is this "user experience" that Mac has and Linux lacks?
Linux, Free and Open Source Software permits to adapt them to your needs, but this shouldn't mean to rediscover the wheel every time!
Deb or Rpm? Pulseaudio, Esound or Alsa? Gtk or Qt? Metacity, Xffm or Compiz? Who cares! The important is the modulariry and interchangeability!
If a lightweight and a full-feature project can't merge their effort at least they should propose a public API to Freedesktop(e.g Xesam), so third-party application should grow up without to be broken to every release.
There are still basic tasks that you MUST use a terminal to understand what is going on and to fix it. The terminal is useful but not easy for the majority of users.
(Different kind of selection highlighning on OS X)
Ubuntu default font is not very nice and many GUIs could be more informative and a gray window isn't a good way to show that the system is busy.
Windows and Linux are used to waste a lot of space for application.
The dock does almost all the things above in one place and the "Application" is displayed ONCE. Linux has Cairo Dock, which is good and very customizable, but in isn't integrated with the rest of the system. (I mean Transmission, Calendar,.. icons change behavior directly in the dock)
Linux has good mature backup tools, rsync, tar, .. but these tools aren't for anybody while the backup is. Flyback it's a nice frontend for rsync but everytime it scans the entire directory structure, so it isn't so smart. Inotify is there, it just should be more used.
It is the only rational way to manage different type of data. A global viewer for Images, Video, pdf, slides,.. and different applications to edit different data in specific tasks.
Gloobus should be integrated in Nautilus in a non-hacky way.
This could be a techie topic but it isn't, anybody would prefere to see a better path for the system files (/lib, /usr/lib, /usr/local/lib, /usr/X11R6/lib) Gobolinux tries to simplify this legacy outdated Unix fashion, but no mainstream distro is going to support it.
A similar situatiation is for user confiiguration files. Where does THIZ app save my config? Try .config/, .gconf/, .app/, .gnome2
All-in-one updating system is nice but it shouldn't provide regression! Ubuntu support proprietary drivers like Nvidia and Ati/fglrx, Intrepid Ibex release broke several working installation causing a regression. This is very bad.
The users should be more cosciensciusness but Ubuntu should advertise better this regression!
See the forums for reference or stay with Hardy Heron LTS as long as possible.