Horizontal Lines

Instead of making a pattern swatch (which often bog down graphics cards if built wrong), just make the design element. 

1. Make a black box (background)

2. Make a smaller grey rectangle.

3. Make a black line or shape and rotate it to desired angle.

4. Copy and paste (or step and repeat / blend tool - just make a bunch of stripes.)

5. Position the first one at the front and the last one at the end.

6. Select all the lines (make sure background is not selected) and align (horizontal distribute center).

Suggest another way to create a diagonal pattern.
It is a method using a pattern brush.

Create the following figure. I use intersect in the pathfinder.

By registering this figure as a pattern brush, you can create the following figure.

With the object selected:

• Hit "R" for "Rotate"
• Move cursor to side of object
• Click and rotate
• Before letting up on cursor hold down "~" key. This will limit rotation to fill only

Create a pattern swatch. Draw a vertical line with whatever stroke you like, and another path with no stroke or fill around it to the width you want the gaps to be, then drag both to the swatches palette:

Draw an object that you want to fill with the pattern and apply the swatch. In the transform palette menu, choose Transform Pattern only, then adjust the rotation: