Here is my suggestion for a rewrite. Changes are marked and commentary
explaining the change is written at the bottom.
GIMP is an acronym for GNU Image Manipulation Program. It is a freely
distributed piece of software that is useful  for graphics-related
tasks  such as photo retouching, image composition and image authoring.
GIMP is extremely capable. While it can be used as a simple paint program,
it is also used  as an expert quality photo retouching program, an
online batch processing system, a mass production image renderer, an image
format converter, and many other functions.
GIMP is extremely  customizable and extensible. The advanced scripting
interface  allows everything from the simplest task to the most complex
image manipulation procedures to be easily scripted. For new  special
effects and other advanced custom features, GIMP is designed so that
plug-ins and extensions can be written to do just about anything.
Plug-ins can also be developed to allow the reading and writing of new
GIMP was originally written and developed under X11 on UNIX platforms, and
the UNIX version is usually the most recent and stable. There are also
OS/2,  MacOS X, and Windows versions available.
Features and Capabilities
Here is a  short list of some of the most important GIMP features. Keep
in mind that this is only the tip of the iceberg.
- Full suite of painting tools including Brush, Pencil, Airbrush, and
- Sub-pixel sampling for all paint tools for high quality anti-aliasing
- Extremely powerful gradient editor and blend tool
- Supports custom brushes and patterns
- Tile based memory managent so image size is limited only by available
- Virtually unlimited number of images may be open at one time 
- Full alpha channel support
- Layers and channels
- Multiple Undo/Redo (limited only by diskspace)
- Transformation tools including rotate, scale, shear and flip
- Selection tools including rectangle, ellipse, free, fuzzy, bezier and
intelligent scissors 
- 'Quickmask' allows the selection to be made by "painting" with paint
- Advanced scripting capabilities
-- A Procedural Database for calling internal GIMP functions from
programs such as in the included Script-Fu application
-- allow for the easy addition of new file formats
-- make it simple to write new effect filters
-- Over 100 plug-ins already available
- Load and save animations in a convenient frame-as-layer format
- File formats supported include gif, jpg, png, xpm, tiff, tga, mpeg, ps,
pdf, pcx, bmp, and many others
- Load, display, convert, save to many file formats
And much, much more! 
1: "suitable" is wimpy sounding, but something is needed
2: general, then with specific examples. Nice.
3: this phrasing illustrates the contrast in the range of uses more
clearly, and also makes it clear that gimp really is used for those
purposes. Ideally, each of these noun phrases should be made into
hyperlinks to places like filmgimp, flamingtext, etc, and to news articles
about how gimp is used in such-and-such.
4: "expandedable"? huh?
5: switched scripting and plug-ins, to go from simple and less powerful to
most complex and powerful. This is how people tend to think, and it saves
the most impressive for last as well.
6: explained why you would want to write a plug-in
7: yeah, I have to hold my nose to use Windows too, but really, can't we
all just get along?
8: even if the list is "thrown together" it doesn't need to say so. :)
9: etc and including were redundant and said the same thing two times.
10: 'may be open'
11: intelligent scissors, not intelligent select. Also added Carol's
12: reformatted this section to use another level of indentation
13: sounds better with the and
Gimp-developer mailing list