Stacks in Aperture2 and Lightroom2 are about the only feature that share some functionality and have the same name. Stacks are a way of grouping similar images together. The images can be various versions of the same image taken in the camera, virtual copies (as termed in LR), versions (same thing but Aperture term), or versions that were modified with an external editor/plug-in. Stacks have the same basic functionality in both applications and can more or less do similar jobs. The magic of stacks comes from their interaction with albums (Aperture) or collections (Lightroom). I will touch a little bit on albums/collections just because the function of images stacks are so closely related to these other features. I will also compare a bit of compare mode functionality as well mostly due to Apertures excellent "stack mode".
First the similarities between Lightroom and Aperture. Both applications can group images together either manually or automatically by the date and time stamps on the original images. They both automatically put new versions/virtual copies in a stack with the original master image. You an move images "up" or "down" in a stack or move an image right to the top. Images at the top of the stack are the images that represent the stack when the stack is closed and there for provide an easy way to select only the images at the top of stacks when all stacks are closed.
The big difference between Aperture2 and Lightroom2 is that stacks are inherent and visible no matter where you are looking at an image in Aperture. When using Lightroom you only have access to stacks and the related stack functionality when looking at images in the library module and then only when clicked on a folder (the closest thing to an Aperture project) view. For instance if you drag an image in a stack into a collection that image shows up in the collection but there is no indication when viewing the collection that it is part of an image stack. There is no stack functionality at all when viewing a collection with one exception. If you create a new virtual copy or version of an image in a collection it will automatically be added to a stack with the original image when viewing that image in a folder. In aperture it is apparent that an image is in a stack no matter where you are looking at it, in an album, a smart album, the project, etc. This may not seem like a big deal but it is huge in terms of a streamlined work-flow. When combined with the functionality of "album picks" in Aperture, the ability for different images to show up at the top of a stack in every different album or album like thing is a huge time saver for creating and managing various versions of an image for different output purposes. Take for example the need for different image crops for standard paper sizes. In Aperture you can close all of your stacks, select all of them (or filter based on whatever criteria you want) create a new album with those. Hit one key to make a new version and then apply an 8x10 crop to all of them at the same time. Just the act of doing this causes the new versions created in the context of the album to show up at the top of the stack in that album. Don't get me wrong there are ways of accomplishing this in Lightroom with much less fanfare than the way we all used to do things like this but Aperture's stacks/stack picks/album picks are major work-flow efficiency tools when used to their best advantage. The Light room equivalent is more like go to the folder, close all the stacks, select all images or filter/select all images, create virtual copies, create new collection, drag collection to collection set, select all images in collection, apply black and white preset. Not a big deal but here is the big deal but longer and clunkier.
The other thing that is different is what is almost a total lack of short cut keys to manipulate stacks in Lightroom. WTF, once you have some experience using an application and you are whizzing through your work-flow using short cut keys and one or two are missing for what you want/need to do it's like hitting a brick wall going 90 mph. The same applies to the lack of short cut keys for Lightroom compare mode. If you get proficient using both Aperture2 compare features and Lightroom features you will feel like you are in slow motion, mouse move point, click, hell in Lightroom. Talking about compare features for a moment, Lightroom has a workable compare mode and just enough keys to make you want the ones it does not have. Lightroom also has absolutely no connection between it's compare features and it's stack features. The big missing key as compared to Aperture is the set compare item key. Don't think this is a big deal? You should have seen the seizures and conniptions that the experienced Aperture users had when Apple removed not that key but the short cut that put you into compare mode and set the compare item at the same time due to newbie users accidently putting themselves in compare mode. The speed of compare mode, short cut keys and stack mode are deal breakers when comparing Aperture2 to Lightroom2 for a lot of people. Stack mode in Aperture2 is like compare mode but tied to image stacks. A summary is that the right/left arrow keys only move the candidate image within the stack, they won't go beyond. The up/down arrow keys move from stack to stack. When changing stacks the "stack pick" or image at the top of the stack is automatically set to the compare images. Last but not least, ta da… there is a shortcut key/function that replaces the compare image with the candidate AND makes it the stack pick.
It's also kind of nice to do a search or use a global smart album and know that an image that shows up is part of an image stack, in other words has other versions or similar shot variations. Even if the other version do not meet the search or smart album there is a grey box around the image along with a badge indicating the number of images in a stack that the image is a part of.
The bottom line stacks, compare mode, and their relationship to versions/virtual copies and collections/albums are workable in Lightroom and better than anything we had a few years ago but Aperture2 blows the living crap out of Lightroom2 when it comes to stack functionality when looked at in the context of an overall work-flow.