For those like me trying to decide between the new Panasonics and Samsungs, here's my two cents after looking at TVs at Best Buy this evening.
There was a 55ST30 and a 59D550 next to each other. The plasma section was limited and they didn't have any higher-end models - I would have preferred to see a 59D6500. I took both TVs off of "torch" mode, putting the Panny on Cinema and the Sammy on Movie. I found Cinema on the Panny to be too dull for my liking, and in that mode it looked lifeless next to the Sammy. I ended up setting the Panny to Custom, which to me was a good balance between the pop of Vivid and the realism of Cinema.
My observations are with those picture modes at their default settings. Changing settings might give different results than what I saw.
Black Level: The ST30 trounced the D550 in black level - it was notably darker. The ST30 reminded me of my parents' Kuro, but not quite as dark. The D550 was more in line with what I'm used to on my current set, a Panasonic TH-50PX600U.
Detail: The Panasonic was much more detailed than the Samsung, with far more information in textures. The feed to the TVs had several closeups of middle-aged and elderly folks. The Panasonic showed every pore, blotch and blemish on their skin, while the Samsung smudged much of that detail out of existence. I actually found the ST30 slightly over-detailed, reproducing the artifacts in the crummy BB feed too well, so I turned it's sharpness down to 25 from the default of 50. This helped, and the image still contained a lot more information than the D550.
Color: Colors seemed a bit more vibrant and involving on the Samsung, without being unrealistic. The colors on the Panasonic were quite good as well. I'll admit I have a bad eye for color, but I found the two sets pretty close in this regard. The Samsung did seem slightly too warm, while the Panasonic was more neutral - but I assume the Samsung could be adjusted to give similar results.
Glare: This is another area were the 55ST30 had a clear advantage over the 59D550. I turned both sets off and looked at my reflection. The Panasonic's anti-glare coating was better than the Samsung's in three ways: First, it gave a slight blur to the reflection, so edges and details were softened. Second, it crushed shadow detail - that is, darker areas of the reflection in the Panny had no detail, while dark areas in the Sammy still had easily-visible details. Third, the Panny's reflection had a somewhat blue tint, while the colors in the reflection on the Sammy were more true-to-life. The blue tint actually helped by making the colors of the reflection duller, more uniform and less noticeable/distracting.
Those are my quick observations of the differences I noticed between the two TVs in the not-ideal in-store setting. I would really have liked to see a D6500 as well, as its supposedly better black levels and anti-glare coating should to some degree address the D550's shortcomings in those areas.
Even though I very much prefer the Samsung's industrial design and would love the additional 4 inches of screen, the Panasonic simply had the better picture. At this point, I would choose the Panasonic ST30 over the Samsung D550. I really want to find a D6500 to demo to see if it can match the ST30.