Laas's face has the same smooth, bloated features as Odo and the Female Changeling. This appearance is not native to their people. Odo's face is a result of his limited abilities as a shapeshifter trying to mimic a Bajoran, while the Female Changeling assumed her appearance to make Odo feel comfortable.
At one point, Laas takes the form of fire. Fire has no matter. A changeling cannot become non-matter.
The prison protests leaving Odo alone with Laas when asked, saying he has his orders, but relents immediately when Kira makes the same request.
Incorrectly regarded as a plot hole: The guard's orders were to not leave Odo alone with Laas, due to a conflict of interest. Kira is not a changeling and wouldn't be held to the same restriction.
Incorrectly regarded as a plot hole: The guard's orders were to not leave Odo alone with Laas, due to a conflict of interest. Kira is not a changeling and wouldn't be held to the same restriction.
Laas shouldn't be able to turn himself into fog, because fog is a collection of water drops. It has been established, that a changelings can only take forms that are one connected mass. This has been used to test characters for being changelings, if a part of their body (blood in that case) were disconnected from the rest it would return to its natural state.
Laas boards the runabout from space. He would not be able to do this without breaching the atmospheric seal on the vessel, as starships must be airtight in order to function in space. Were the ship breached in such a fashion, it would trigger a warning from the ship's computer, which is not heard. Additionally, a starship is constantly surrounding by a deflector shield (to protect it from space debris), which it is also impossible to breach without triggering a warning.
When Laas first enters the runabout, O'Brien draws a phaser from his waist. Starfleet personnel only carry phasers during combat situations. O'Brien and Odo were on their way back from a conference and were in friendly space. O'Brien had been asleep just moments before entering the cockpit. There would be no cause for him to have a phaser on his person, other than to draw it on the intruder at the tail end of the scene.
Laas is arrested and held in a holding cell. If he could board the runabout, he could get himself out of a holding cell. He clearly has no regard for humanoids or their laws.
Odo says Laas cannot be a Founder because he is not infected with the morphogenic virus that has infected the Great Link. He says that he asked Dr. Bashir to scan Laas's morphogenic matrix and it is, "as stable as mine." Ignoring that Bashir (or anyone in Starfleet) has yet to encounter an infected changeling and may not know what to look for, Odo is later revealed to be infected with the virus. Thus, Laas's matrix being as stable as Odo's is not a sign that Laas is not infected, as he could have the virus but it may have only progressed as far as Odo's.
When asked by Bashir and O'Brien to do something about Laas, who has assumed the form of a dense layer of fog around the promenade, Odo says that Laas isn't hurting anyone. Fog that thick would create, at the very least, tripping hazards. Odo is usually a stickler for such things.
Laas berates Odo for not linking with him on the promenade, saying Odo must be embarrassed to be seen for what he truly is by the humanoid inhabitants of the station. Even among changeling, linking is considered and intimate act. Two changelings linking in public would be akin to two humanoids engaging in sex in public. As Laas has linked with Odo and gained understanding of linking from him (who in turn gained understanding of it from the Female Changeling), he should understand this.
Odo restrains a Klingon who'd just stabbed Laas with a knife, allowing Laas to stab the Klingon's compatriot. Changelings cannot be harmed by bladed weapons. Odo's reaction shows very poor threat assessment.