Was Coulson dead or near-dead? Is Tahiti a metaphor?

I was under the impression that Coulsen was a LMD (Life Model Decoy).

This is a plot device that has been used before in the comics (Nick Fury often turns out to be a LMD whenever he is defeated) and is even mentioned in Iron Man III:

"You have reached the life model decoy of Tony Stark, please leave a message..."

This would explain his ongoing memory lapses, the alluded-to 'loss of muscle memory', and the number of times an old companion or colleague has mentioned that he 'doesn't seem the same'.

Of course, this is merely speculation on my part (and many others), but the signs all seem to be pointing that way.

As for Tahiti - implanted memories?

To quote Clark Gregg (who plays Agent Coulson):

What’s the deal with Tahiti. Tahiti’s a magical place. I keep saying that… it’s where I went to recover after being impaled and as I say in the pilot, it was a spectacular experience, I had a beautiful physical therapist and I read Travis McGee novels all day.

But every time someone mentions it lately I seem to go into post traumatic stress so I think there’s a little more to Tahiti than meets the eye.

and

There’s a much darker, more interesting story there. Something much more magical has gone on to bring this guy back to life.

Because of the word "magical" one theory is that Scarlet Witch had something to do with his resurrection. (Scarlet Witch will appear in 'The Avengers 2'). But at the moment it's pure speculation.

In an interview (Nov 18, 2013) producer Jed Whedon had this to say about Coulson's resurrection:

We know that there's going to be a lot of theories and there's no chance that whatever we reveal won't, at some point, be written on the internet. We're just trying to come up with ways to make it surprising or make it not how they expected it to unravel.

We know what it is, but we know that no one else does yet.

The last sentence is interesting, because it implies that the most popular theories, which Jed Whedon must be aware of, are probably wrong.

From what I gather from watching the first four episodes, so far, it seems that Coulson's death and return will be a part of the story arc for the show. It appears that Coulson did indeed die and was somehow brought back, possbly using alien (Asgardian, Chitauri, other) technology.

Coulson himself believes he was near death and was sent to Tahiti to recuperate. From dialog between Melinda May and Grant Ward in one of the first episodes, it appears that it's accepted knowledge that that did not happen.

I'm guessing whatever happened to him is behind Nick Fury's decision to give him this team, and will be revealed, slowly, throughout the season.

At a certain point in the first season of the show, it was revealed that Coulson died for a lot longer than the number of seconds he was led to believe he died for.

He actually underwent an experimental procedure named Project T.A.H.I.T.I, which was designed to resurrect a member of the Avengers in the event of their death using the blood of an alien from the Kree species. Nick Fury decided that Coulson was so important to the creation of the Avengers that his death warranted the use of the procedure too.

The reason that Coulson can't remember any of this is that the process of bringing people back to life also had the side-effect of implanting the Kree's "genetic memory" into them, sending the test subjects mostly insane and compelling them to carve strange alien symbols and diagrams into whatever they could get their hands on.

As such, a memory wipe became part of the procedure in order to attempt to remove the implanted "genetic memory" - however, unbeknownst to SHIELD, that only ever worked temporarily. After seeing how traumatic the overall process was, the person running the tests shut the project down - in an ironic twist, that would be Agent Phil Coulson himself. Whose memories of the project were deliberately wiped when he was bought back to life using the very project he had tried to stop. His memories of this period were replaced with - you guessed it - a relaxing holiday in Tahiti. It's such a magical place.

In short, Coulson was bought back to life by an experimental procedure that he personally disagreed with, which is why its use was kept secret from him.