In particular I love the folder concept that has cleared up my screen and made it so much easier to find applications because of grouping like with like. Limited multi-tasking, too, means switching between frequently used applications is almost instantaneous. Background apps with notifications or GPS activity (such as TomTom) can run away with battery power, but the remedy is easy once you realise what's happening.
If I'm asked, though, what is the best improvement in iOS4 it must be keyboard support. The iPad dock keyboard perfectly complements the iPhone, even the 3GS, and increases productivity tenfold. With the phone docked, the keyboard looks just right – better, in fact than when it hosts the relatively massive bulk of the iPad.
I'm now using the dock keyboard and iPhone routinely for tackling serious work when out and about. The iPad, increasingly, remains at home and does what it does best – acting as a reader for books, newspapers and magazines. However, the iPad dock keyboard does have one big disadvantage in bulk. The fixed bracket to hold the device makes it awkward to pack. I have little doubt that we will soon see a new version with a folding bracket/stand. Then we will have the perfect travelling companion. Who needs a laptop or an iPad?