Tags are supposed to make finding related articles easier by quickly summing up the contents.
So when tagging, you should try to „sum up in zero to four words what the article is about“. (including „zero“ and „four“)
cities and places (köln, hamburg,…) if the article is about an event / erfa / … in that place.
places outside of Germany: also tag with country (?) +1
if it's about an erfa, chaostreff, … or an event organized by them: put the „official“ abbreviation of their name as a tag (cccmz, cccs, c3ma, …)
names of companies and products (siemens, …)
names of events; if there's an established abbreviation, use it (28c3, mrmcd, …)
titles of other $things (c-radar, …)
names of persons (first name + last name or last name only?) +1 I'd suggest first+last because some last names are rather common
titles and names of projects (blinkenlights, …)
articles containing a link to a video should be tagged with „video“.
what about pictures?
Not 100% sure. The idea behind the „video“ tag was that users who are reading using a cellphone (limited bandwidth, small screen) or can't use headphones or loudspeakers at the moment for some reason can skip over articles which require watching a video (which often relies on an audio track as well) to get the message across.
I propose: Tag articles with images with „pic“, and articles with videos with „video“, if and only if the picture / video is the *main content* of the article. If the message is delivered using plain text, and the picutre / video is just a supplement, don't use the picture / video tag.