Titles should be completely independent of tags. A good title tells an expert whether they want to read the full question all by itself.
I know this has come up multiple times before, before, but in each case, the discussion has seemed to have turned into a different question. For example,
Should questions include "tags" in their titles? really asks, "Don't we need tags in titles to help with SEO"?
So, "no, we don't, because we add the top tag (or two, in some cases) to the crawler results," is correct, but it appears that people are interpreting that to mean that people generally can (or should) omit key terms from their titles if they exist in the tags.
To be clear:
I agree that we should never force a tag into a title if it's not helpful; Why didn't my beer cake cook properly? should NOT have "[Baking]:" jammed into the title, because it adds nothing to a reader's understanding of what it's about, BUT...
Nothing should be left out of a title just because it exists in the tags.
Trilogy examples: How do I make a machine "blank screen" for a period of time (as a penalty) if certain noise levels are reached?, What is the purpose of the holes marked "Do Not Cover" on hard drives?, What's wrong with always being root?
Arqade examples: https://gaming.stackexchange.com/questions/68710/how-is-experience-divided-in-multiplayer, What actions reward experience?, Ways to earn experience-points,
The problem with all of these is that without a language, OS, or game title, they fail to convey whether you care to read the question, because you need to read the tags to even figure out if you work with the language or own the game in question.
A term being in a tag in no way reduces the need for it in the title. Tags are great for filtering, but a title is supposed to function like a headline.
This is a good headline:
This is not very good headline:
Titles, like headlines, are BIG AND BOLD on the front and other pages. So you can browse and stuff. Tags are small, and kind of low contrast. So they don't, you know, distract you from the headlines.
Bear in mind that on non-technical sites, new visitors, even new experts, don't necessarily even know what tags are.
The title test:
If you assume the reader can't even see the tags, is your title a good, stand-alone summary of the question for an expert on that topic?
I'm sticking with gaming examples because they're more broadly accessible than some topics:
Why do zerglings with speed beat normal zerglings? does not need, "in Starcraft" added, because it's impossible to be interested in the answer and not know that a zergling question relates to Starcraft.
Besides practicing, what are some great ways to become a better player?, absolutely needs "...in Starcraft 2" added to it, because without the tags, it's currently useless as a way to decide if you want to read it.
This is really no change from Jeff Atwood's comment:
In the wake of that comment, people seemed to mostly hear:
"stop forcing a word you don't need in just cuz it's a tag"
... which is right, but what it really says is,
"most of the time, some tags will naturally belong in a title that effectively summarizes the question".
EDIT: Some folks felt this was (an unduly public) way to call out gaming specifically. It's not - I think this is a problem to varying degrees on a number of sites, but used examples from gaming because they're easy for most readers to follow. Sincere apologies to anyone in the Arqade community who felt I was using a network-wide forum to criticize a gaming practice - I wanted to clarify the best practice for all the sites that encounter this at least sometimes, including SO, where you still see people stripping languages from titles in edits sometimes.
"Why do zerglings with speed beat normal zerglings? does not need 'in Starcraft' added"- if someone makes a popular-enough game also featuring zerglings (which might even be a sequel) to which the question doesn't apply or has a different answer, will that make you change your opinion on this? That would then seem like a flawed premise. This also seems like a flawed premise on the basis that it works for everyone who knows Starcraft, but not for anyone who doesn't.