what makes sense so great?

Sense, specifically Sense 2.0, is an efficient visually based UI that adds a lot of functionality that isn't necessarily there.

 

Sense is just a user interface that works on top/with android. It's something HTC built to work on top of and with android, both to add missing or better features, as well as give a visually more appealing presentation.

 

Motoblur looks to do the same thing, as does touchwiz for samsung, but they are not nearly as feature rich, and have been known to bog down as the device ages. Most original droid owners have rooted away motoblur for this particular reason, while many original HTC Hero owners still have sense on their devices because it's functional (though admittedly sense 1 was not nearly as streamlined as the new second generation sense UI is).

It's all about personal preference in the end, but if you are looking at it from a visual and feature perspective, sense offers a ton more functionality than any of the other custom manufacturer UI's.

 

Andoid is the operating system. Motorola use MOTO BLUR, HTC uses Sense and others may use other veneers, as the Doorway or User Interface into Android. These manufacturers may add certain special widgets or ease of use features etc. Clocks on the home screen, special menus Social Feeds ...

In addition, some manufacturers have taken Android 2.1 (eclair) and added some featues that are on Android 2.2 (froyo) to bring their devices similar to 2.2 without actually using 2.2. That is the case with Blur and Sense

Of course this is an actual 2.2 device...