It doesn't get much easier than "
apt-get install <package>" -- it'll work out what other packages are needed from your entire gamut of repositories, let you know what it's going to change, tell you if it can't proceed for some reason, all the important stuff. It might not be "pretty", but by gum it's superbly functional, and as long as you don't go and force it to do something it says is a bad idea (or install severely broken packages), it's really, really difficult to break the system.
It also centralizes update services. When a new version of any program is available from your gamut of repositories, it detects this and will offer to upgrade your software. No need for vendors to duplicate upgrade services over and over again, flooding a system tray with Java updaters, Flash updaters, Acrobat updaters, Quicktime updaters, eclipse updaters, etc. It's better to have one high profile and well reviewed tool to handle this than leave it up to each software provider to perform the complicated process of updating their program.
Finally, the apt design supports a flexible set of transport protocols. If you desire a new system to acquire packages and updates you're free to implement the handler without throwing away existing functionality. I've seen people implement torrent transports for packages, to offload mirroring costs to a cloud of users, for example.