I looked at the flyer, but it wasn't entirely obvious what it was advertising.

I had an idea ages ago (after seeing some similarly minimally-informational flyers, possibly a teaser campaign i.e. with further detail to be provided later) that it would be funny to put up flyers for Christmas as if it was some sort of event. You know -- "CHRISTMAS. Dec 25th. 00:01 - 23:59."

Increasingly I'm finding Wikipedia fills this niche for me

I find Google's "define:" can be quite handy if what I need is a quick "what's this all about?" definition. (Tested it on 'UML' and it gave me several answers that sounded reasonably lucid and sensible, though probably not enough detail for what you want.)

Particularly bad is if the link goes via one of those URL-squashing services which removes any chance of you being able to look at the URL itself in advance and know whether it's a news site or a comedy site or what.

Guilty: I admit I've often done that deliberately because otherwise the surprise of the picture/film/mp3/whatever would be spoilt by the URL. ("Hey, that reminds me of something I saw the other day: http://www.made-up-website.com/kitten_trapped_in_photocopier.jpg") OTOH I do try to say at least specify "funny thing" or "requires flash/sound" or "(N)SFW".

I just think that writing, say, ten words or so to give people the first idea of what something is shouldn't be that difficult for anybody

I would normally have agreed that it should be easy (and I think in some cases it is) but it's just taken me about an hour to put together a pitiful amount of words (maybe 500?) on what $thing is, why people should give a damn, and where they can find out more (including 'here's a link, this is what's at the other end of it'). It's particularly disconcerting to be trying to write about something that Wikipedia doesn't seem to have an article about, as that's normally where I steal ideas get my inspiration.