If you're planning to listen to music on the Apple Watch while you go running, you may need to keep your expectations in check. Apple tells 9to5Mac that the wearable has 8GB of built-in storage (twice as much as a typical Android Wear watch), but that there are set limits on how much media you can store. Only 2GB is available for music, so don't think of stuffing in a gigantic exercise playlist. In practice, you're looking at about 200 to 250 songs at the 256Kbps quality you get from the iTunes Store. There's also just 75MB of space for photos, although that won't matter as much given that the Watch both limits you to favorite photos and resizes images to match the smaller display.
So what's the remaining 6GB of space for, then? Some of it is likely devoted to the operating system and app caches, but the rest is likely reserved for what's coming down the line. Right now, all Apple Watch apps are just extensions of iPhone software -- while storage isn't a problem at the moment, you'll want considerably more of it when native apps arrive. That lack of flexibility could be frustrating if you want an eclectic song mix, but at least you'll know why you can't fill your Watch to the brim.