Normally I wouldn't think this card is overpowered or anything but when comparing to Bamboo Watchtower I see a major disconnect. It requires no domain, and only 1 more flux than watchtower for 3 more health. It also has a special ability, but I can ignore that since it seems rather weak.

I think this is just a result of the fact that flux costs are discrete. 2 flux is too little for a 2 vision building. 3 might be a little high, but there doesn't seem to be any room for middle ground with the watchtower. Simiarly, 4 flux might be a little low for the Sphere, but 5 is definitely too high since for all practical purposes it has no ability (I think I have used it exactly once in all the time I have been using them).

It has always seemed a little odd that it doesn't require any domain though. That always struck me about trunk hermit too.

Sphere of radiance is ok, not that it really sees much use. Domain buildings in general have no domain requirements.
The problem lies in the bamboo watchtower, and its not really a big huge problem, its just a victim of granularity.
Though truth be told i have totally stopped using bamboos, even beacons in most sylvan decks (even pure sylvan ones). Bottom line is the 4 vision to the next closest flux well costs upwards of 7+ flux and 2 ss domain. This is much better acomplished with Fairy+Spire or the enchantment-hopping palace. Or even mahal bullshit.

It's not really a domain building though, since if you ever get domain from it you must somehow have had elemental domain to cast an elemental creature (unless you stole one from your opponent). I think it ought to be seeing more use. A 2 vision building with no domain requirement is a nice fit for many decks.

Today, 2 spheres of radiance and a vapor mahal let me control the center flux well in a game by round 2 It was glorious.

sphere of radiance is a good card, and it's a bit weird that it doesn't even require domain, but really the difference between 3 flux and 4 flux is bigger than it seems.