There are a number of reasons I believe this to be the case:
1. The detail is sharp, crisp, and plentiful. We're talking raised buttons, smooth cloth waves and folds, and plenty of differentiation between peices. There are some exceptions (around the cartridge boxes and boyonet scabbards), but in general the raised details are obvious and therefore easy for even a beginning painter like myself to pick out.
2. Head options galore! Each kit comes with a ton of heads. The British have options for tricorns, left side cocked, jockey caps, and grenadiers. The Continentals get tricorns, jockey caps, "Tarleton" helmets (for making Washington's Life Guard), and uncocked felt. It looks like the Militia box (which I don't yet own) also has several options including knit stocking caps. by mixing and matching these options you can create a HUGE number of combinations between all of the kits.
4. Color instruction sheets. Each kit comes with a color coded sheet that shows you which pieces are used for every pose. For instance the drummer might be all of the "M" bits on the sprue, and the picture would show them all as, let's say green. That makes it simple to find them on the sprue and to make sure I'm not missing anything. Oh yeah, and the bits are labeled on the sprue as well. ;)
I obviously think these kits are amazing, but I also feel that they're perfect for someone who is interesting in playing Muskets and Tomahawks. The continental kit alone can make over 300 points of an American force. Throw in a militia box for not only militia but also riflemen and you have a diverse and LARGE force that is quick to assemble (for plastics) and quick to paint.
Here's a few more WIP Brits just so you can get an idea of the variety from each box. Remember there's only a single box of British troops out, but you can build so many units!