How to deal with "too descriptive" comment in dissertation proposal feedback?

The difference between descriptive writing and critical writing is much like the difference between a newspaper report and an opinion column.

Descriptive writing is the act of reporting on what's in the literature:

Smith found that when X occurred Y and Z also happened.

Critical writing analyzes what has been done, and takes note of trends, as well as possibly offering feedback on the overall quality of the research:

Smith found X occurred in the presence of Y and Z, as did Jones. However, Doe has demonstrated that Y and Z normally occur in conjunction with one another, so it is not clear if X actually influences Y and Z, or if it is an independent effect.

What you can see from the above is that critical writing does require some descriptive writing, but it goes well beyond it in terms of the depth of analysis.

To extend aeismail's comment slightly, a good critical analysis will not only summarize and critique findings, but also extrapolate conclusions from multiple studies to support/disprove current theories. Each individual paper provides evidence to some small piece of the overall puzzle; a good review will tie together many related (and some unrelated) papers to build an argument towards a general conclusion, using the individual research findings as support for their argument.

Note that the individual results are almost expected to contradict each other. You can frequently have a batch of papers supporting theory A, and a second set of papers supporting theory B. This should be noted and identified as well; these are the current trends in your field of research.

I would like to tackle this question from another angle for you.

You have done a literature review for the dissertation proposal. You report your findings from those literature after you study them, so you have descriptive writings in the proposal.

But, what do you propose to research? Have you discovered anything? Have you challenged the existing literature? What do you want to research? Why do you want to research that topic? What are your arguments? What would be your own approaches? Why would your approach work? Etc.

Those are the things they are looking for. They want you to have critical edge in the proposal.

I found a link from University of Leicester about What is critical writing very helpful, at least to me. It contains several sections, What is critical writing? What is descriptive writing? The difference between descriptive writing and critical writing, examples and other useful information.