Pardon me, but can't you tweak all these in JPEG too? I thought after the file is open in Photoshop, it does not really matter anymore since JPEG also have to be decompressed to raw data before Photoshop can display them. Sorry, that's what I understand all along so just want to clarify.
I also did not understand this point when I first heard about RAW. But after playing with RAW, I find I prefer the picture quality when I shoot in RAW compared with JPEG. RAW pictures hold the information in a photo better than JPEG pictures. Tweaking a photo in RAW, you may consider it to have 100% information after you've converted. That's as much information as you are going to get out your RAW file with those settings.
Whatever you do to your jpeg picture in photoshop, you are losing information (in fact, any picture format that is edited, you lose information whether editing a GIF, BMP, JPEG, TIFF etc...). Every time you do leveling, or curves, or colour balancing, your picture is getting less and less information in it. This is why you should try to do all your photo-shopping on the original picture, and not on photos that have already been edited. When you adjust photos in RAW, and then convert to JPEG, for example changing the exposure, or colour balance, you are not losing information as such. The software is processing the raw data that came out of sensor according to a different set of parameters chosen by you.
Your photos can all tolerate a certain amount of post-processing. If you shoot in RAW, you can withstand the most postprocessing. If you shoot in JPEG, you have much less ability to withstand tweaking.
As has been pointed out, JPEG is a lossy format, ie: information is lost when you save it as a jpeg. The designers of jpeg tried to optimize the information that is lost, and they've done a pretty good job. Additional information is lost when you perform your photoshop functions (curves, levels, balancing etc...) regardless of the format you save in. When you adjust the exposure in photoshop, the photo has less information after than it did before. When you adjust the exposure in RAW, the information has been processed differently. Thus you may consider that the information has not been lost.
Because of this, you can under or over expose photos by 3 stops and still get a useable picture if you shoot in RAW (although it won't look as good as if you got the exposure right). If you under or over expose a photo by 3 stops and you're saving to jpeg, you are going to have a much harder time getting a useable picture out of that, and it may indeed not be possible. If it is possible, your resultant photo will be very fragile, in that any further editing may quickly render it unuseable.
It comes down to how much post-processing your pictures will need to tolerate. If you shoot perfect exposure and never need any post-processing, you may as well shoot in jpeg. If you anticipate needing some or lots of post-processing, then shoot in RAW. I always need postprocessing, so I always shoot in RAW.