I'm a pro photographer and I have just a few things to say
1/ keep it lightweight, avoid the temptation to eat CPU% and RAM with bloatware nobody really needs
you have a nicely styled intuitive interface that kicks the cr@p out of PS legendary 'where do you get it' cloistered menus
Lighweight processing engines are fast, and people appreciate that more than a hit list of features that marketers prefer.
If I spend 2 mins processing an image in a day, thats 200 mins or 3.3 hours processing I never get paid for.
Reducing that 25% is a big and important deal. This is why I still use PSP X instead of newer versions
2/ one of the greatest utilities about PSP interface is using the mouse scroll wheel to zoom in and out of an image,
do not ever let that go it is the single best thing about PSP bar none
3/ another thing to maintain is .8bf compatibility, it helps you keep your software capability up to date when the supplied tools begin to look a little old,
I use things like Imagenomic NR and Nik sharpener, when the next best plugin arrives I shall probably get that too
4/ another thought, RAW conversions via an .8bf plugin,
in this way you can do some trade with PS users who are being skinned alive by Adobe over their update pricing policy
Everytime a photographer gets a new camera (usually because he must), he has to wait for RAW support to catch up,
and probably has to buy the upgrade vers of an application just to stay current with his/her competitors
5/ You guys would be well served offering a Mac capable version of your software, check this thread out
see how old it is, how people obviously found it via a search engine, and actually joined the board just to have their say
http://forums.macrumors.com/showthread.php?t=167822
honourable long term users there going to waste looking for an alternative on mac platforms,
.........and not finding it,
I would have gone mac years ago if I could take PSP with me, sad sad sad