I think it's a combination of better hardware and carrier improvements.
Check the link in my sig, people are saying that going to the 4s they are getting a lot better service now on AT&T than before the 4s
Well, I don't believe we owe AT&T an apology, but neither do they owe us one. Neither AT&T nor Apple EVER expected the original iPhone to take off like it did, and they simply weren't ready for it on their network. So since the other carriers have seen the data usage that they can expect, they have had time to make needed upgrades so that the customer experience is better. However, saying that, Sprint has not done so well by me. I have a 3G Aircard that I've used for almost 2 years, and until Friday, I was getting about 500-700k downloads. Since then, I have not gotten above 120k! Once my contract is up, it goes away.
I have to agree with WannaGoMac on the improvements. When my wife would call me on my 3GS, she complained that she couldn't hear me, I couldn't understand her half the time, but now, with my new 4S, same calls from same places, she has not complained once! I hear her much better, and she claims to hear me better as well. The HSPA+ I get at my house is a huge benefit as well. I also need the ability to look things up while on the phone, so AT&T has been the only choice. I get about 5-8MB downloads all the time, so have nothing to complain about there.