TYPO3 Core

You can already do this with typoscript like this:

10 = TEXTAREA
10 {
    cols = 40
    name = comment
    rows = 5
    data = default value
}

But "data" can not be rendered as contentelement.
I pushed a patch that make this possible.

With this patch you can do something like this:

10 = TEXTAREA
10 {
    cols = 40
    name = comment
    rows = 5
    data = TEXT
    data.value = x
}

For documentation:

"data" can be used with TEXTAREA, TEXTBLOCK and OPTION.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/42685

You can already do this with typoscript like this:

10 = TEXTAREA
10 {
    cols = 40
    name = comment
    rows = 5
    data = default value
}

The rendering as contentelement is a security risk, because backend users are able to execute custom typoscript.

As it already has been pointed out - data is used instead of value for the Form Objects TEXTAREA, TEXTBLOCK and OPTION
Thus, I suggest to deprecated data for those three object and use value as default value - as it is being used for all other objects.