You can't do layer masks in LR, but you can do some limited changes to specific spots in a picture.
I use both LR3 and CS4. The two are complimentary but have different strengths and weaknesses.
One of the things I like is how you can apply the same settings to multiple pictures. For instance, if you have a series of pictures all with the same wrong white balance, all slightly underexposed and all slightly crooked. Make as many changes as you need on one picture, select all of the rest and you can apply all those changes to the rest, at once. You can also apply only some of those changes (i.e., white balance, profile assignment and exposure but not straightening because they aren't all crooked).
It sounds like assigning tags can be similar to PSE - select all the pictures you want to assign keywords to and do it at once. You can also assign key words when you import them into LR also.
I won't get rid of either program, I prefer CS4 for resizing and sharpening. Also for anything that needs layer masks or different layers using a different mode.