The way the signal bars are being calculated is what they are fixing. So, whereas before you had to have over 80% (for example) to get 5 bars, now you have to have over 95%. That's an oversimplification, but hopefully you get the idea.
Plucking numbers out of the air to try to illustrate this, the calculation currently uses
1 bar - 5-10%
2 bars - 11-20%
3 bars - 21-30%
4 bars - 31-40%
5 bars - 41% +
The attenuation of the signal from the "death grip" (sheesh) is still there (as it is with all phones), but the signal strength indicator is being calculated more appropriately. For example, if you were in an area with a signal strength of 41%, you would see 5 bars. If cupping it in your hand then reduced the signal strength by 21%, then you would see it drop to 2 bars. Holy cow! A drop of 3 bars. If your signal strength was at 90% and you cupped it in your hand (which drops the signal strength by 21%) then it would drop to 69%. But this would still display as 5 bars.
This is why some people are seeing the problem and some aren't. It all depends on the signal strength before touching the phone. All phones are affected by this effect, but if they are reporting the signal strength differently then it won't be seen as a problem. What's to say that another phone isn't displaying 5 bars for anything over 15% signal strength!
Now, if the code that displays the bars is tied to the other code in the system, then a low signal strength could trigger the phone to try to swap to 2G network, or to try to negotiate a new connection, etc. So fixing the bars displayed may result in a better operation.
The change to the height of the bars is simply to make it easier to see and is just a "while we are making the other changes, then we might as well change that".
I fully expect that the apple-haters and iPhone-bashers out there will completely ignore these facts and carry on bleating on about how the "iFail" (or whatever other stupid thing they come up with to call it) is so rubbish and that their xxxx phone is far superior in every respect. Blah blah.