The series 1 watch is essentially identical to the original Apple watch, although Apple upgraded the chip to the new dual-core S2 chip to make it a bit faster. I bought a used original Apple watch recently, and it works well for the most part. I've read others state that the upgraded chip isn't too different from the original; it's simply marginally faster. Battery life has also been improved from the original Apple watch
The original Sport Apple Watches also had a composite back that covered the heart-rate sensor as opposed to the ceramic coating of the regular Apple watch model. All series 1 and series 2 models now have this ceramic coating, which helps keep the heart rate sensor area pristine. (Edit: as noted by LiveM, all series 2 watches including the sport model have the ceramic coating, but the Series 1 does not.)
The Series 2 watches have the same dual-core s2 chip, as well as the improved battery life. They are slightly thicker than the original, and they are now water-resistant up to 50 meters. The original Apple watch and Series 1 are only rated for up to 3 meters of depth for a maximum of 30 minutes, although they aren't recommended to be submerged; they're more splash-proof. The series 2 watches also have built-in GPS to allow them to track your runs without the need to have your phone with you. The series 2 also has a screen thats twice as bright as the series 1 and original watch.
The Apple bands are easily interchangeable and are all of good quality; I like most of them. The sport bands come with an extra strap in a different length if your wrist is larger or smaller for a better fit.
As of now, Apple isn't allowing 3rd party watch faces, although their standard ones are all customizable and allow you to change colors, how simplistic you want them to look, and you can also add "complications" which allow you to launch some applications quickly or provide you with micro information at a glance such as the weather, battery life, or the date for instance.
I really like the watch, although it's not for everyone. It does take some adjusting of the settings to set which notifications you actually want to receive. You may not want to be bugged constantly with trivial notifications. The watch does not provide you as much usefulness as an iPhone though, so don't expect anything life changing.
The workout features on the watch are pretty good, so if you exercise or want something to motivate you to do so, the Apple watch is nice to have. I would also say to expect a much smaller ecosystem of third party apps for the watch. It's primarily a device to notify you about stuff and allow you to tell the time, set reminders and timers, and maybe allow you to quickly control things such as music or other simple things. I don't really use any third party apps to be honest.
