The thing I have the most issues with is his ability to manipulate his mass. He can be as light as a glass on a tray, or as heavy as an 80-kg human. You canna change the laws of physics, captain.
Why not? Everybody else in Trek manipulates mass willy-nilly. All it takes is a gadget. For a Changeling, the gadget may be an organ - perhaps the only organ they possess at that.
Also, he can imitate a rat or German Shepherd with absolute precision. So, why not a humanoid face?
He sort of covers that in "Paradise Lost", by saying that his apparently perfect seagull (that is, a real seagull!) wouldn't convince a fellow seagull.
WIth humans, faces
are hard to do. The best CGI creations may otherwise fool us, but even the most complex fake face money can buy will look slightly off. With seagulls, faces probably don't matter: birds can't do facial expressions, and their brains aren't bred to recognize those. But perfect seagull body language may in turn be far beyond Odo's skills.
With dogs... Well, again, facial expressions wouldn't matter much. But dogs are visual, and can learn to read the facial expressions of more expressive species such as humans. Then again, Odo never said he couldn't do a perfect German Shepherd. But since he very well knows he can't do a perfect Bajoran face, he probably accepts that there's no point in even trying: the uncanny valley would be worse than what he gets by doing a rigid mask.
That would mean that there's an upper mass limit to what he can morph into, when he has none of his usable mass left in subspace. Probably that would be true for the "Odo" form, or otherwise we'd have to assume that that part of him got split off when he was changed into a Solid by the other founders.(Assuming that they left it 'in store' for him just to cover for the possibility he'd eventually become a changeling at some point in the future again seems a bit far fetched to me). That in turn would mean he cannot change into something heavier than Odo, e.g. a 500 kg boulder, or a "genuine" elephant.
Unless, of course, the real Odo weighs in at thirty tons.
We know that when knocked unconscious, Odo is "heavier than he looks", as in "Vortex". Perhaps completely comatose, he would weigh even more - such as when he's locked in a stasis chamber in "Invasive Procedures"? We just couldn't tell, because even at thirty tons, Odo probably wouldn't fall through space station decks or even stasis chamber shelves.
Does Odo shunt additional
matter into Changelingspace, or does he merely shunt the
mass of that matter? Since Odo can become anything, according to a tricorder, probably the latter: otherwise he'd need to keep X tons of every possible type of matter in reserve, rather than just genuine goo that can assume required properties (such as mass) when called for. And if Odo shunts mass, we don't really need to think there would be a limit: he could have a trillion quintillion tons in his Changelingspace pocket, or then a gram, all of it equally meaningless because mass there doesn't have any pull or crack any linoleum on the Changelingspace floors.
Timo Saloniemi