Lightroom and Photoshop aren't really (supposed to be) competing products -- they're complementary products that overlap. Lightroom is all about managing your workflow, which just happens to include image processing in most cases. Photoshop, on the other hand, is image processing to the max, but it doesn't really do much to help your workflow. There's very little you can do in Lightroom that you can't do in Photoshop, therefore, but if you're working on a lot of photos, you'll probably find that Lightroom saves you quite a bit of time.

The most common workflow for both of these products together uses Lightroom to import photos, then tag and cull them for further processing (or vice versa). A fair bit of general image processing can be done right in Lightroom, but for images that you really want to spend some time on, you can shoot those photos over to Photoshop for heavy-duty processing and (optionally) pull the results back into Lightroom.

For areas where these products overlap (RAW processing, etc.), they're generally either equivalent or in some cases actually use the same binary bits, so if you don't think you need the workflow portions of Lightroom, you shouldn't miss out on any of its image processing features. For what it's worth, people who value the workflow stuff in Lightroom but don't need to tap all the power of Photoshop can often get by with Lightroom plus Photoshop Elements -- that's a great combo, too.