What are the differences between major Linux distributions? Will I notice?

I think two main difference is

• Package management
• Default Window Manager (Gnome, KDE, XFCE)

Just pointing to these two big difference will somewhat change your behaviour in doing task. For example you could easily install a package on Ubuntu using apt but not so simple when using rpm. Having a difference in Window Manager can also change how you interact with the system.

There are two major flavors of Linux distros out there. Debian and Red Hat based distributions account for a large percentage of the distributions out there.

The major difference between them is the package management. If they are Debian based, they most likely use the dpkg& apt/deb system; if it's a Red Hat system, it most likely uses yum/rpm. A lot of distros pop up because someone was unsatisfied with the package management, so most will have some form of graphical interface that is different, but the underlying system is the same.

If you learn how to use apt-get and yum, you'll cover 80% of the distros out there, and 99% of the systems you'll most likely encounter.

Each distro will do something a little different with the window manager. Most major distros choose between KDE and Gnome, with Gnome seeming to be the popular one at the moment. The great thing about Linux though, is that you can change the window manager if you like (go Window Maker!).

If you learn the command line, there is not "much" of a difference, but the GUI changes between distros will definitely sway your choice. Also, some distros ship with drivers that will make it easy for certain hardware like video cards.

I highly recommend Linux Mint for home systems, and vanilla Debian, Ubuntu (server edition), or CentOS for servers.

Even though the package managers are different, my understanding (though I don't use Linux regularly) is that most packages are available some way or another with any distro. Also, the window systems / desktop environments are different, but you can install alternates anyway. The only other thing I can think of right now would be the simple difference in communities surrounding distros.

I'm pretty sure that you'll be able to find a way to do whatever you want to do.