In the Nitpicker's Guides, Phil Farrand pointed out Odo can pull off a rat well enough to fool anyone, but can't manage a nose. He said he thought Odo wasn't trying very hard. Me, I think Odo's difficulty is a PSYCHOLOGICAL one, not a physical one. For some reason, he thinks of himself so specifically as "other" when dealing with humanoids that he can't copy any of them well- his subconscious sabotages the attempts. Although, I think, by the series finale, he'd get pretty close to a good copy if he tried- because his subconscious had changed. (We also know that, had he lived long enough, he'd work it out- we saw that with the planet with the alternate timeline, when the Defiant crashed in the past and the survivors made a society.