The most significant, and overlooked item for many developers is properly setting up your DNS. "It it ain't broke, don't fix it" - it may not look broke, but when you start really looking, it can be hosing up (quote!) your web site's visibility, your email, and kicking your stress level up a notch:

Have you checked your DNS lately? [webmasterworld.com]

If you do eCommerce, you shouldn't tap another keyboard stroke without reading the input from this thread (and others like it) discussing customer reactions to logging in on checkout:

Logging in: do people hate it [webmasterworld.com]?

I've begun hanging out more at what should be one of the most important forums for web developers. For starters,

Target Lawsuit Ends with $6 million Settlement [webmasterworld.com]

... should be an eye-opener for developers that consider accessibility an issue they'll deal with "when I get to it."

Same forum, P1R brings up yet another Important Point that Very Few Seem to Consider:

Be Specific, Use Fragment Identifiers [webmasterworld.com]

[edited by: encyclo at 11:14 am (utc) on Dec. 23, 2008]

Logging in - Do people hate [webmasterworld.com]

Always a good thread when you get the lorax "**TWWEEETT!**" to stop the discussion. I forgot how good that debate was, funny thing is, I still don't know which way I would sway.

In addition to the ones listed above, I found the following worth of flagging.

A list of guaranteed techniques to make your business fail for free.
How to make a business fail [webmasterworld.com]

This thread came a little late for me, a while after I realized the core points of the discussion, but used to soldify the stand I have taken for a while.
Vast Ideas [webmasterworld.com]

Tables - What is so bad about them again? [webmasterworld.com]

The little things that make you say WOW! [webmasterworld.com]

Nearly all the best threads make it to the Library here [webmasterworld.com]

[edited by: Habtom at 4:45 am (utc) on Dec. 24, 2008]

best of 2008 from the XML forum:

SOAP, REST, or MySQL? [webmasterworld.com] (July)

Google announces Protocol Buffers [webmasterworld.com] (July)

Are XML and JSON homeomorphic? [webmasterworld.com] (March)

XML is 10 [webmasterworld.com] (February)

Some of the CSS threads:

• Why "variables" in CSS are harmful [webmasterworld.com] and the somewhat related Microsoft to end expressions in new release of ie [webmasterworld.com].
  
• Slicing images, how does it differ using div's and floats vs tables [webmasterworld.com], but we also had solutions for those insisting on slicing the background image [webmasterworld.com] anyway.
  
• During the year we had a few tutorial style threads showing the journey to create CSS, taking the problem of a member as a goal. The first of them was here [webmasterworld.com]. We had a more philosophical discussion [webmasterworld.com] on the subject as well.
  

As always, "best" is in the eye of the beholder.

Not necessarily the best from Foo but among those that generated the most interest. Yes I know, sometimes Fooites can build an entire thread around one word. :)

In descending order from newest to oldest:

How To Make a Business Fail
[webmasterworld.com...]

Comcast Bandwidth Cap
[webmasterworld.com...]

If WW Allowed Sig Lines
[webmasterworld.com...]

Am I Making Too Much Out of This?
[webmasterworld.com...]

British Telecom Ad Replacement Trial
[webmasterworld.com...]

Google Street View Not Welcome Everywhere
[webmasterworld.com...]

Bug Crawling In My Screen
[webmasterworld.com...]

Phrases and Accents That Drive You Mad
[webmasterworld.com...]

I Neutered My Neighbor's Cat
[webmasterworld.com...]

Are Bloggers the New Press Reporters?
[webmasterworld.com...]

Annoying Driving Habits
[webmasterworld.com...]

Overused Words and Phrases
[webmasterworld.com...]

Another "Word" Thread
[webmasterworld.com...]

[edited by: phranque at 9:15 am (utc) on Dec. 28, 2008]
[edit reason] fix url [/edit]

i started my list by looking at all the most active threads (>30 posts during 2008) in Webmaster General and immediately realized that made a most worthy list, so in chronological order with credit to the OP:
'Hacker Safe' Site Hacked! [webmasterworld.com] by walkman
Why is using tables for layout evil? [webmasterworld.com] by Duskrider
AT&T Says Internet's Current Architecture Will Reach Its Limit By 2010 [webmasterworld.com] by engine
Creating a Multi-Lingual Website [webmasterworld.com] by teamcoltra
Free Dynamic DNS Services Pose Massive Security Threats [webmasterworld.com] by incrediBILL
The Web is Increasingly a Girls World [webmasterworld.com] by martinibuster
Massive Botnet Spamming Infestation Thwarts Standard Detection [webmasterworld.com] by incrediBILL
Theplanet's data centre in Houston catches fire [webmasterworld.com] by dusky
Is there a future for CAPTCHA as it is now? [webmasterworld.com] by eelixduppy
Hackers May Be Targeting Google Gadgets [webmasterworld.com] by engine
Lawyers Using Your Own Web Site Against You [webmasterworld.com] by incrediBILL
Internet Anonymity Being Reviewed By U.N. Agency With IP Traceback Initiative [webmasterworld.com] by engine
The "Webmaster" Job Title [webmasterworld.com] by rcshield
Violating TOS now legally the same as hacking [webmasterworld.com] by seth_wilde
as you can see there is a heavy influence by fellow moderators here.

and for winner of WebGen thread of the year 2008, i recursively nominate this thread:
Best WebmasterWorld Threads of 2008 [webmasterworld.com] by jeyKay

besides the WebGen threads i also wanted to highlight what i thought were some of the most important threads regarding the philosophy that semantics provides accessibility which provides seo benefits.
this list obviously relies heavily on the Accessibility and Usability forum as well as the Search Engine Promotion forum.
for the sake of sanity i stayed away from Google Search threads for this list.
with all due respect to encyclo's bible and especially to pageoneresults' tireless evangelizing in this area:
Are Accessibility and Usability Synonymous with SEO?:
[webmasterworld.com...]
No More WWW:
[webmasterworld.com...]
26 Steps to a More Accessible Website:
[webmasterworld.com...]
If you can read this...:
[webmasterworld.com...]
The Title Attribute:
[webmasterworld.com...]
Be Specific, Use Fragment Identifiers:
[webmasterworld.com...]
No More Extensions:
[webmasterworld.com...]
The tel URI for Telephone Numbers:
[webmasterworld.com...]
Web Content Accessibility Guidelines (WCAG) 2.0:
[webmasterworld.com...]
The ADDRESS Element:
[webmasterworld.com...]
The Ultimate SEO Guide for 2009:
[webmasterworld.com...]

Why Affiliates Refuse to Make Money
[webmasterworld.com...]

How To Secure Wordpress - several iterations of this (I wonder why) but this is a good start:
[webmasterworld.com...]