For me, one of the biggest advantages of Lightroom over Photoshop is the ability to use Develop Presets on your images.
You can create several virtual copies of an image and use different Develop Presets to see how they come out. Doing so uses virtually no extra hard drive space as the changes are stored in the Lightroom Catalog in the form of text commands.
The Fader plug-in is the best bet you could hope for. When you need an Opacity Slider without leaving the LIghtroom program, you will need this program. Read further on in the article to see how you can make the transition simplier.