Photoshop vs. GIMP
Many people in the open source community claim that GIMP can do everything that Photoshop can do. Unfortunately, along with ignoring the learning curve presented with a completely new menu layout, there are some important features missing from GIMP that more advanced users simply cannot live without.
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Learning a new workflow. Many serious artists, such as my wife for instance, are simply not willing to bother with this. They are perfectly happy to use a commercial product like Adobe's Photoshop instead. Casual users are often hung up here as well, as they do not enjoy relearning to work with a new photo editor.
Lack of commercial-level features. The two biggest lacking features happen to be a lack of support for CMYK and PANTONE. Despite the average user never seeing this come up for their simple edits, advanced users are not likely to be this forgiving.
While there are serious open source applications in this field that are used by pros, like Inkscape and Cinepaint, GIMP is not taken as seriously in the professional world as Photoshop.
Overcoming learning curves and a general perception of application difficulty.
Believe it or not, most of the previously mentioned challenges are in fact hype driven. Here is a fact check:
The average person can migrate to Open Office easily, it just means taking a little time with the user interface and expecting some migration-based format issues with existing documents.
If a user is not familiar with Photoshop, using GIMP is not going to be all that more difficult to learn. It simply takes time to get used to finding the tools and finding a workflow that works best for that user.
Most popular open source applications have books available to make the switch a lot more painless from the user's standpoint.
With proper education about open source alternatives to piracy, encouraging realistic user expectations of what they are getting for their time invested in learning, I suspect that there will be a noticeable positive shift in the piracy wars.
One example of progress in this area would be with Linux distributions. Linux distros are beginning to chip away at the piracy problem with Microsoft Windows. But for some closed source software companies, this may be a path they do not want to see well traveled given the eventual loss of their market share.