When you open the crop tool in Lightroom 4 and CS6 you will immediately notice changes, some of which may throw you for a short while. For one, the crop guidelines immediately embrace the entire image and you may modify the image using the corner or edge handles simply by dragging them into the image. Adding the shift key constrains the crop to the original image aspect ratio, and shift and alt/option keys together keep the crop centered so that the edges all move together.
In the upper left on the options bar you have the ability to set this behavior as well and also to choose an aspect ratio for a specific output as you are likely accustomed to doing in the past in ACR and Lightroom. If you drag a corner to get the sze you want, the crop window may not encompass the image as you wish it to be. This is fixed by simply releasing the mouse from the handles and you can now move the image, not the crop guide, so that the part of the image you wish to use is positioned inside of the guide.
The crop guide itself includes options for assisting you in making compositional decisions with overlays in several including the Rule of Thirds, a Grid, the Golden Ratio and several others. You can, of course, turn these off of put them into “auto” mode in which case they appear when you grab a handle and disappear when you let go. Other options include automatically centering the crop view, showing or hiding the cropped off original pixels and reverting the tool to the legacy operation you are used to. I recommend you live with the tool for a while as you may discover you like the new options once you get accustomed to them.
While the enter key commits your crop as you would expect, Photoshop has added cancel, restore and commit icons similar to older Elements style which looks a bit silly to me. Anyone working in Photoshop for more than a little while understands the Esc and Enter keys well enough that the icons are really not necessary.
Within the crop options in LR4 is a straighten option which is also in the Options bar in Photoshop, and in both programs it can also be activated by holding the Control key down. Probably moving the image instead of the crop box is the biggest thing to get accustomed to, and it makes sense pretty quickly.
While the legacy option is there, it is also possible to simply put your cursor where you want to start a crop and drag to where you want to end within an image and the aspect ratio in play will be respected when you do this. That means you can pretty much work as you did in the past without having to relearn the tool. All in all, some advantages and added options to the tool that can be useful.
Missing from the options bar, but available in the drop down for aspect ratios is a box to add resolution to the mix. In CS6 hitting the R key with the crop tool active opens the resolution box so you can plug in the numbers you need. The X key rotates the crop from vertical to horizontal if you need to, but that behavior still works as you move the cursor within the image changing the crop area.
One big change is the “Delete Cropped Pixels” check-box in the Options bar in CS6. The default is unchecked which gives you the option to change your mind on a crop later in the process. Changing the aspect ratio or grabbing the crop handles and the original image content reappears so you are not stuck with your original decision. This works with saved PSD files which says to me that the image is being treated as a smart object by default. Once you flatten the file you lose this option.