Like many others, I come from a background of heavy GIMP/Photoshop/Photoshop Elements use and I do a lot with photos especially with correction and retouch.
There are some tools that are SORELY missing in this area and yes there might be ways around it but still hurts immensely when you are used to other simpler ways of getting things done.
I will go over the tools/filters that I feel need to be added and then go over some improvements to some existing ones.
No Average blur filter. Some may love and some may hate it but this tool is very useful to help remove a huge color cast on a photo. A lot less painful than sliding around color balance sliders I can say that!
No White Balance tool. This again is a love/hate tool but it is very weird that the iOS version of this program has it but not the big brother on the Mac.
No color sampling tool. No I don't mean the eyedropper. I mean a tool that will place a point on the image/canvas and allow one to see what the colors are under that point. I use these to help color correct by the numbers. I know that Pixelmator will allow you to see the color under the pointer however this is NOT practical when doing color correction by the numbers.
As far as the existing tools/filters, I see that the levels will only adjust the overall brightness and not any of the channels. You have the channels programmed into the curves filter/dialog, wonder why it isn't in levels even though you have the histogram visible in the tool?
I know that this will most likely start a flame war in here and that is not my intent! Sure if I have a lot of time and I know what is supposed to be what color, I can fiddle with he color balance tool however sometimes that isn't the case and because I do not have access to the channels (Oops, another feature I guess) of the image, I cannot take out a glaring color cast without a ton of fooling around.
You could also use channel mixer as well but again why kill yourself when some of the above will easily do what is needed?
Please don't get me wrong, I'm really liking Pixelmator! Still learning where everything is and how to use it but sometimes I have to go back to another editor because of these tools not being here.
Thank you and I look forward to what the future holds for this program!