Inspired by a newbie on alt.fan.pratchett complaining that she hadn't been welcomed properly[0] ...
A few (well, several) afpisms, which I've added in footnotes to explain. I'd advise opening up two copies and scrolling one to the footnotes so you can read them in parallel.
Was rather pleased with this one :)

Welcome In

Welcome in, welcome in
Let the fun and games begin
Pull a chair up, have some chocolate, [1]
Get accustomed to the din
Have a drink -
Heck, have two
Ask the fairies for a clue [2]
Get the tagging system mastered [3]
Spit, and call the cat a bastard [1]
Speak of cheese [4]
Speak of bread [4]
You could even start a thread
(Though remember - not to tag would be a sin) [3]
So get into that queue, be
An outspoken newbie,
Welcome in, 
Hell - come *in*! 
Welcome in

LSpace links [5]
Helpful FAQs [6]
You can find them here in stacks [7]
We don't bite (although we might
If asked politely), so relax
Think us strange
Think us weird
Get yourself a hat-and-beard [8]
Eat and drink, and yes, be merry
Wonder why we call him 'Pterry' [9]
Talk in verse
Talk in rings
Talk in random-looking strings
With an acronymphomaniacal grin [10]
Insert a footnote *there* - [11]
'Twill add a touch of flair -
And welcome in
Take a spin
Round our little loony bin
Welcome in, welcome in, welcome in!

Life can get quite boring for a newbie we're ignoring [12]
Ill at ease, trying not to freeze, out in the rain
Oh, the tragedy of an existence
With your nose against the windowpane

Days without a greeting, not a soul so much as meeting
The proprieties, extending you a hand
Sometimes we just need to be reminded...
Ask why we didn't do it and behold - we snap right to it!

Welcome in!
Welcome in!
Conjure up an evil twin
Get a sig (but not too big) [13]
Attempt to figure out your kin [14]
Learn to quote
Learn to trim
(There's a HOWTO you can skim
If your quoting style's in doubt - look
For the postings headed 'Outlook' [15]
It's a pain, it's a mess
You must fix - but I digress -
(Though I really would consider using tin))
Heck - there are even kooks[1] [16]
Who talk about the books
So welcome in, welcome in, welcome in
Yes, welcome in!

[1] eye rhyme, but it made a nice ending.

Here be footnotes...
[0] newbies tend, in general, to be pounced upon and greeted with great enthusiasm. this one slipped through the cracks due to delurking silently (i.e. just joining in, without a 'hi everyone! i'm new')
[1] The current choice of greeting is 'pull up a chair, have some chocolate and call the cat a bastard', or some variant thereof. The first two are traditional, the last is newish and derives from Bertram Chandler's 'This is liberty hall. Spit on the mat and call the cat a bastard'
[2] 'clue fairies' - indefatigable suppliers of clue to bewildered newbies. basically, the people to ask if you need something explained.
[3] due to the high volume of posts on afp, subject lines are required to begin with a tag - [R]elevant, [I]rrelevant, [A]nnotation and a few others.
[4] perennial favourite topics
[5] www.lspace.org
[6] posted regularly to the group, or search [5]
[7] what's life without a g&s reference
[8] current injoke about the number of beard-and-hatted patrons.
[9] after names like Pteppic and Ptraci in Pyramids
[10] 'acronymphomania' is the faq what explains all the acronyms and abbreviations
[11] footnotes are a somewhat popular embellishment to afposts :)
[12] entire verse in reference to said newbie's 'no one greeted me' post
[13] very keen on the 72x4 limit, afpers are
[14] reference to the rather complicated network of 'afprelations' (afpfiance, afpsis, etc. - just another piece of afpsilliness)
[15] periodic posts explaining to Outlook Express users how to get around the annoying brokennesses in said 'newsreader'
[16] footnote from filk