The Illustrator live line segment tool draws lines using the current stroke setting. You can find it in the tools panel.
The key panel for this is the properties panel (window menu). Select, set the stroke weight, set color, click art board, draw using it. You can vary the stroke etc at any point such as adding arrowheads such as circular arrowheads. It can be applied multiple times. It generates two points that can be modified in 1000s of ways using such as direct selection and curvature. It can be used to create dramatic artworks in seconds.
It is also live and can be edited via the Illustrator properties panel. You can change the angle, position, start point, fill, stroke, angle, opacity and more. Weirdly, you have to expand the properties (...) to access the length option which you would think is a more useful feature.
You can also add arrowheads via the stroke panel. You can add an arrowhead to the start and end of the path and there are a variety of different shapes. You can increase the size of the arrowhead via the scale feature in the strokes as well as by changing the width of the path. A selection below, they can be on both ends. Set the arrow via the stroke section.
You can at any time turn the stroke open path into a closed filled path via the object menu and expand. You can no longer modify it via the properties panel though.
You can also modify the artwork by adding dashes, by clicking the dashed option on the properties. The dashed design can be altered in a number of combinations such as different combos of dash and gap as well as using different cap and corner and alignments.
You can also vary how the designs ends (as well as how the dashes look) by changing the cap settings in the strokes as well as the alignment (inside, outside etc)
You can add an artwork and the width is the same all along the path " 20pt or 40pt etc but you can also apply a profile to this design, go the control bar and select a width profile from the drop down to apply or use the width to change the width at certain points along the path.
I generally go to the preferences section and set up the preferences for the smart guide to include most of the features. Smart guides can be very useful as well as baffling at times in that sometimes where you expect a smart guide you won't see it and in some places you will. I am certain the smart guides will grow and be added to over and over in the next few versions of AI and they are generally worth checking out. Set the transformation to on and construction guides. You can use them without those set on as well.
You can select the end points and rotate and as you move the artwork you will see smart guides popping up saying certain distances from certain paths and not others as well as various alignments. You may or may not like that feature but personally, I think the info is useful and it is great to be able to select a certain point to stop moving the path or changing the angle. The reference point does not seem to have any use. To extend the length of the path just select either end point and stretch out to the required point or go the other way to shrink the path. You can also change the angle via the transformations panel as well as change the length and position of the path via the top bar. Create a number of vertical designs, select the top point of first, hold the shift key, drag the end point up or down, repeat with the other paths by selecting the top end point
You can also use the horizontal setting (change the reference point to be the middle bottom)
The rotate can be done as before by using the selection and having the control just close to the end point (above). You can constrain the angle of the rotation to 45 degrees etc. Create a path, select the top end point, rotate, hold shift to constrain. Or use the transform panel to rotate the path
You can apply the it with the tilde / grave held down. What happens, well, movement of the mouse will generate a vast number of paths (some short, some long) all depending on the origin point and the current position of the cursor. You can go back and overlap the paths as well or just stop by releasing the mouse click.
You can still change the stroke width, add width profiles (which is quite useful with the stretch and shrink end points) as well as brushes, effects, appearances, styles etc and the path is still a live shape. Actually, unlike some of the other live features, it does seem that there is no way to break the live aspect so all the effects are changed as the length is changed. Select, create a path, set the stroke weight, click stroke to display stroke panel, click dashed to ON, set dash to 5 and 10 etc. Or select the path and goto the brushes and select a brush stroke which can then be modified by double clicking the brushes setting
You can add to the design via the appearance panel, you can go to the right side menu and add new fills as well as new strokes and also duplicate existing items and then change the settings such as stroke width as well as changing the color of the entry. You can also add into the mix for each entry an effect or two (effect menu) such as offset to offset the stroke (so you can see it)
You will see the generated path listed as a "line" and not a path in the layers. You can also use the fact that is a different shape and use the selection to select the same shape option to capture all the identical types though it would be nice if there was a select rectangles and select ellipses etc feature as well (sadly, not). Create the artwork, go to the layers panel, expand layer entry, see design displayed in the listing (not path etc)
Use the Illustrator expand command but then it just becomes a fill. The solution is to go to the object menu and select the shape option and expand (you can also do the reverse with the convert command). You can, of course, just avoid the it altogether and use the pen o create two points (always useful to have a number of features to create the same sort of thing)
You can add effects to the artwork so you can add a blur or transform etc and you can vary these via the appearance panel at a later point. You can add multiple effects to the artwork, radically changing the design by using some of the features such as zigzag etc
You can use the artwork with the repeat feature via the object menu and repeat and radial, simply select the design and generate your radial design, you can increase the number of items via the interactive controls, you can interactively change the angle of the artwork etc. You can also double click on the repeat object and then modify the source artwork at any point.