Myself and plenty of other people would love to get away from Photoshop, but without this feature, it's simply not an option.
I've tried to switch from Photoshop to Pixelmator, Acorn, Draw It, Gimp and Incscape - but no dice. Photoshop still has the best features for pixel design.
The problem with catering to photographer is probably rather that they already have an awesome, lightweight and modern Photoshop replacement – even one made by Adobe – called Lightroom (and also Aperture by Apple).
There aren't that many really professional photographers out there (i.e. people who do it for a living) but there are masses of hobbyists.
There are plenty of people for whom Photoshop Elements or iPhoto are too simple and for whom Photoshop is overkill. Those people might be tempted to get Pixelmator but both Aperture and Lightroom seem like a much much sweeter deal for them to me.
Lightroom does some excellent workflow management but it also does some (quite complex, actually) photo editing.
Lightroom does do some complex whole-image photo editing, but it still sucks for doing any kind of localized changes. Everyone I know who is serious about photography uses both Lightroom and Photoshop. As mentioned upthread Photoshop can be skipped for the vast majority of photos where you're just cropping, dealing with white balance and doing subtle adjustments, but when you need to make real edits to the contents of the photos, Lightroom doesn't come close to replacing the tools Photoshop has for that.
Original responder was right, Lightroom isn't a drop-in replacement for Photoshop and most people still use both. People who are just doing very light photo editing and could get by without Photoshop could also probably get by without Lightroom and just use Picasa.
I do wish Lightroom added some more of the hardcore content editing features Photoshop has because I'd love to get them into the fully non-destructive editing stack (Lightroom's killer feature), but it likely will never step too much on Photoshop's toes because of Adobe's own version of "the Microsoft tax".
No, Lightroom is obviously no drop in replacement for Photoshop. Photoshop can do different stuff. Conversely, Photoshop is no drop in replacement for Lightroom. Not in a thousand years. Photoshop sucks as a Lightroom replacement, more than Lightroom as a Photoshop replacement.
And you can do localized edits with Lightroom. In the context of Photoshop being fucking expensive I think that’s kinda relevant. Photoshop is that one very specialized tool and you can get by without it.
Read his original PhD thesis on "PatchMatch: A Randomized Correspondence Algorithm for Structural Image Editing" http://gfx.cs.princeton.edu/pubs/Barnes_2009_PAR/index.php
As well as the follow up paper with collaboration from Adobe: "Video Tapestries with Continuous Temporal Zoom" (published at SIGGRAPH) http://gfx.cs.princeton.edu/pubs/Barnes_2010_VTW/index.php
But I think the question is really strange. It's like asking "What is the Porsche sedan lacking that the Volvo sedan provides?"
They're for different people. Photoshop in my mind is for professionals who use a lot of different tools to master their craft. While Pixelmator seems much more focused on photo-editing and some quick work.
Pixelmator makes design and (more advanced than iPhoto-) photo-editing more easily available to non-professionals.
If you really use everything Photoshop has to offer, I don't think the price is unreasonable. But few people do.
it doesn't get more basic than that imho :)
I've had Pixelmator since it came out. I use it, Acorn, and Photoshop Elements. I would really like to get off the Adobe bandwagon since Elements has insidiously overbearing DRM where you get two installs and you lose an install permanently every time your hard drive goes down, so that you weren't able to delicense first. So I had Elements installed on my laptop and my desktop and when the laptop's drive kicked the bucket, then I was down to one activation and could no longer use it on the laptop without buying it again. This is annoying and it is not how my hammer works. My hammer works whether or not my hard drive has gone down. It's also not how a lot of other software works. But it is how some works.
So Pixelmator has the Apple Store thing. One install only, so none of this nonsense of installing on laptop and desktop even though both are used by the same single person who never uses both at once. However, if the harddrive goes down, there is some way to get the license back, delicensing is not necessary first.
But in any case I don't want to buy Apple Store stuff because I don't want my machine ID tied to my credit card number with Apple. Apple is doing too much privacy invading and I want nothing to do with it. If others want that fine.
So Pixelmator 2 will be Apple store only, and no upgrade price for loyal Pixelmator 1 users.
So that's it. Not upgrading and not recommending it to anyone.
That leaves Acorn and Gimp.
Photoshop is hands down a better program than anything else, but the last few years it seems most of their time has been spent on DRM. Photoshop is constantly trying to connect to the internet, which it has absolutely no legitimate reason that benefits the customer to be doing. Programs that connect to the internet that have no reason to are intrinsically suspicious.
I can see more of the value proposition for open source because of all this DRM stuff.
I pay for all my software. I am the good customer. But over the top DRM is driving me to alternatives, forcing me to have to use open source solutions instead even if I'd rather have something that's a little more polished like a commercial product.
Oh well. I am fine using the alternatives. It's interesting to me though that companies would do this. Such as take an app, pull it from distribution unless you lock your hardware to a credit card at an Apple store. Why would I want to do that just to use a image editing app?
Sure for many people this is no big deal. They don't care about privacy or fair use of things they have bought. But some of us do.
App Store terms say: "You can install apps on every Mac authorised for your personal use and even download them again." 
Also, you can create an App Store account using only gift cards that you can pay in cash. Apple will still know your IP address, of course.
Apps are not deleted or deactivated if someone with a different Apple ID logs into the Mac App Store.
Apps are available to all user accounts on a Mac, no matter which one bought the app. The Apple ID doesn’t have to re-entered if an app someone else downloaded is opened†.
Apps someone else bought will not stop working after they are copied from one Mac to another, no prompt for the Apple ID appears†.
The Apple ID is necessary for updates∆.
Legally you are allowed to put your apps on all the Macs you own or control for personal use (i.e. not the Macs at your workplace or school if personal use is not allowed on them), though nothing is preventing you from putting them on every Mac you want to, either by copying† or by re-downloading in the Mac App Store.
A credit card is not required for creating an Apple ID. (It is possible to pay with gift cards. In some countries alternate means of payment might also be available.) Apple’s terms of service do, however, require you to be truthful when you sign up – you are legally not allowed to enter a fake name or address.
In my opinion Pixelmator’s pre-App Store DRM was less or about equally permissive. You used to have to enter a serial number, now you have to enter your Apple ID (and I wasn’t even asked for my Apple ID when I was just copying apps around, see †). I’m not sure whether Pixelmator’s license was quite as permissive. (Being able to install apps on all Macs you own or control for personal use is certainly generous.)
As for privacy, if you want to pay with a credit card you have to give your data to someone. Either some payment processor like PayPal or directly to the app developers or – now – to Apple. I fail to see the big difference and, as I said, you don’t even have to use a credit card.
Is there some reason that makes you believe that Apple is less trustful than whatever payment processor Pixelmator used before?
† Apple’s help documents claim that users may be asked for the Apple ID associated with the purchase after first running the app on a different Mac. In my own testing I was never asked for an Apple ID. I tried several different apps – free and not – with several different Apple IDs on two different Macs.
∆ I’m not sure whether you have to enter the Apple ID of the account that bought the app or whether it is sufficient to simply enter a valid Apple ID and I have no means of testing that right now.