Of all the functionality that Cpanel offers, what are the most important, most used tools?

Offhand, I would say probably the One-Click scripts.

Fantastico is great, but I wish it was free or was included with cPanel :(

Could you elaborate? I have seen Cpanel, but not used it extensively. What is good about these features?

There are scripts like bulletin boards, or e-commerce scripts which you could install with few clicks in CPanel, without looking for them, downloading, unpacking then configuring.

That sounds like a really cool feature, but would you really say it's the most important or most used? What features do people rely on daily or weekly to administer their accounts?

What do you define as tools ? I would say the email configurations :) if you consider that a tool.

All aspects of email and website administering in cpanel are important. They make people's lives a lot easier as you don't need root access to control the various parts of your domain and its email.

right -- what features are most used, what is most important. I assumed email configuraiton was probably on the list... what else?

cPanel is a very user-friendly control panel, even for the novice. So without focusing on any specific tools, overall it is pretty intuitive. Generally more so than other control panels.

As well, Fantastico is in fact very handy. Although, Ensim is catching up nicely with Ensim Power Tools. And actually, if you look at the Power Tools, they have a few options that are over and above what Fantastico offers.

Vito

a number of users also use the file manager a bit. It's still a bit buggy sometimes (example when you try to chmod)

I don't quite consider installers well used. In fact, it mostly encourage users to do trial installs, play with it for a couple of minutes, and it's left to rot...

People that I've introduced cPanel to were quite fond of it's e-mail handling capabilities, pre-installed CGI scripts, and visitor tracking programs.

I agree with Vito that cPanel is a very intuitive piece of software. I personally like using it because I am able to manage many aspects of my sites through the control panel itself, which is also very easy to use.

Generally speaking, I think the best approach to marketing cPanel is to put emphasis on how user-friendly it is. I would think that the reason why most people want a control panel in the first place is because it allows them to do every-day things in a more efficient, controlled, and simple manner.

Well I am sorry I took this post in the wrong way. I thought by features you meant things that make it unique from the other control panels. I took it for granted here, but Email, MySQL, and Sub/Addon/Parked Domains are all fairly vital features to be used.

As Synthetic said, the strength of cPanel lies in it's user friendly interface and skins. Everything seems (at least to me and many of the customers I have talked with about cPanel) to flow in a logical manner that requires a small amount of work to both find and perform the desired task.