It's friendly in appearance because it combines the classic elements of familiar typefaces that have been in use from more than 100 years like Clarendon, Century, Cheltenham and Clearface. The rounded letters (b, c, d, e, o, p, q) are a bit squarish on the inside. This feature opens up the counters for better rendering and also make it look a bit more up-to-date than the classic typefaces previously referenced.

The serifs are a bit shorter than usual. Another feature that improves the rendering by allowing more "air" between each letter pair.

The joins of the stems to the branches in letters like h, m, n are deep enough to prevent dark spots, also improving legibility at small sizes.

The friendly lowercase 'a', with the curve starting from the bottom of the stem, is reminiscent of Cheltenham and Clearface. That soft curve is also echoed in the curves of the f, j, n, m and r.
The spacing is also optimized for body text on the web, clearly more open than that of typefaces made for print or for headlines.

Open Sans is a humanist sans serif typeface designed by Steve Matteson, Type Director of Ascender Corp. This version contains the complete 897 character set, which includes the standard ISO Latin 1, Latin CE, Greek and Cyrillic character sets. Open Sans was designed with an upright stress, open forms and a neutral, yet friendly appearance. It was optimized for print, web, and mobile interfaces, and has excellent legibility characteristics in its letterforms.

One morning, when Gregor Samsa woke from troubled dreams, he found himself transformed in his bed into a horrible vermin. He lay on his armour-like back, and if he lifted his head a little he could see his brown belly, slightly domed and divided by arches into stiff sections. The bedding was hardly able to cover it and seemed ready to slide off any moment. His many legs, pitifully thin compared with the size of the rest of him, waved about helplessly as he looked.

"What's happened to me?" he thought. It wasn't a dream. His room, a proper human room although a little too small, lay peacefully between its four familiar walls. A collection of textile samples lay spread out on the table - Samsa was a travelling salesman - and above it there hung a picture that he had recently cut out of an illustrated magazine and housed in a nice, gilded frame. It showed a lady fitted out with a fur hat and fur boa who sat upright, raising a heavy fur muff that covered the whole of her lower arm towards the viewer.

Read the rest of this lovely book here.