Line break are used widely on wikipedia. They present information in a tidy and comprehensible way. Comma separation is by no means more accepted among wikipedians. Take a look at other media related infoboxes like the movie, television program, actor, etc. infoboxes and you'll see line breaks are prevalent (see User:Kameejl/Infobox_analysis). Even the musical artist infobox is not consistent in its use of separators; commas are used for labels, but members are almost always line break separated.
The main purpose of an infobox is to quickly inform a reader so the reader immidiatelly sees he is on the right page. Information should be presented in a rapidly comprehensable way. Lists organize things in an easily readable and digestible format. Lists of entities such as associated act, band members and genres are quickly understandable, and cannot contain any ambiguities.
Non-lists such as comma separated entities can lead to problems such as:
Entities consisting of multiple words (f.e. Alternative rock, Crosby, Stills, Nash & Young, Absolutely Kosher Records) can cut in two parts at the end of the content's space. This decreases scanning speed and readability. The use of the {{nowrap}} template should fix this problem but its use brings along new problems:
Keep it simple: many contributors don't know {{nowrap}}
When {{nowrap}} and other templates are used together rendering problems may occur. See this example.
Rows contain an unfixed amount of entries. Some rows contain 2 f.e. genres, some rows contain 3 or 1, even 2,5 or 1,5 is possible (see bullet above). This inconsistency makes article scanning not evident.
Many possible (visual) delimiters like commas or semicolons have a punctuational purpose in prose. Using these kinds of delimiters could lead to ambiguous situations and decrease readability. Who are Nash & Young?
"Wall of text" syndrome: seperating many entities by punctuation marks can lead to prose like text where the separate entities cannot be distinguished easily. Look up The Dirty Mac:
Non-line break delimited content uses the whole width allocated for that content. If this allocated width is not fixed, and can be changed by other factors (such as image or logo width in infoboxes), then the lay-out of the non-line break delimited content can be influenced.
Related to last point: text based browsers (which people may use for accessibility reasons) and other browsers that don't support Cascading Style Sheets use the whole screen width to display comma separated content. (See Lynx (browser) result [1] and Firefox result [2]). This looks inconstent with other infobox content and prohibits fast top-down page/infobox scanning (see #Research).
A list is the most simple and straight forward way to represent an enumeration of entities. Lists are all around us; think of shopping lists, road signs, book indexes, news websites etc. On artist and band pages lists are used for sections like discographies, band members, external links, references. Lists are the way to go if you want a smoothly readable and comprehensible article.
Usability research (conducted with eye-tracking devices) has proven people in general "love lists". People tend to scan pages from top to bottom and don't like prose-like text when scanning a page for valuable information. "Walls of text" can be avoided by using such widespread design techniques as (bulleted) lists. Good use of whitespace and lists improve a readers understanding of an article. Please read [3].
