Are liberals never satisfied?

In a word, no, which is a good thing in the sense that it keeps things going forward, another reason we're also called progressives.  Obviously it's a "bad" thing when it makes you always look at the glass as being half empty instead of half full, which is paradoxical to me because I feel liberals overall are optimists, not pessimists.
This discussion is prompted by Jonathan Chait's provocative article in New York Magazine, Debunking Obama's So-Called Leadership Failure.  While the article is not completely taking liberals to task --

Okay, so if Obama openly endorses a bipartisan plan, he’s killing it. And if he keeps his distance, he’s also killing it. What if he tries to directly negotiate a deficit reduction plan behind closes doors? Well, Obama did that, too, this last summer. Republicans opposed it as well.

-- it does most certainly do that.  "Various fiscal scolds have been scolding President Obama for failing to use his mind-control powers to force Republicans to accept a tax hike."

And it is true that many liberals look too fondly at historic Democratic Presidents, seemingly forgetting the warts and the fact that at the time we were just as critical and not satisfied with that President as we are with President Obama today.  Now, to be fair, that particular predilection is not owned by Democrats.  Republicans too have a tendency to view past Republican Presidents this way.  The difference is they view current Republican Presidents with the same rose colored glasses.
I think what's useful about this discussion is an effort in determining whether it is overall a good thing or bad thing that liberals are generally never satisfied.  I don't think there's much debate about whether we are, in fact, never satisfied, though you're certainly entitled to disagree.  What results from our tendency to view things the way we do is a much more accurate view of reality.  We tend to care very little (especially us Independents) whether there's a D or an R in front of your name; we will scrutinize your words and actions about as fairly as possible, given that all human beings have a lean towards their personal biases.

Also Jonathan Chait was interviewed regarding his article by Chris Matthews on Hardball along with Salon's Joan Walsh; and Joan Walsh makes a very good statement regarding our values about 3/4 of the way through this interview.  It's a lively and fun poke at ourselves, well worth watching.  (Chris doesn't interrupt as much as he usually does)
 

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So I agree, we're never satisfied but I don't think that at all means we're not mostly reasonable.  Everyone's unreasonable sometimes.