Upon impression, newborn dragons announce their name, and speak in human language in their rider's mind. The automatic response might be similar to how many animals instinctively display to each other in greeting or when searching for mates. It is my theory that they are capable of this by drawing from their partner's knowledge, rather than being genetically programmed to speak English by Kitti Ping. I suspect a dragon would speak the language of it's impressor, no matter who they were. It only makes sense that if dragons speak with their rider's mental voice, as mentioned in the books, that it's due to a shared mind, and therefore shared language and memory. This is also why dragons can have only a 3 day memory, but they can tap into their rider's memories when then need to for experience and wisdom. The need to impress to complete their functioning mind is also supported by the fact that they die soon after hatching if they fail to find a compatible human. In the naming of the dragon, it is safe to take inspiration from something that would be significant in the subconscious in the rider, or recent in their thoughts before impressing. A hint could be in the names of some canon dragons: Jaxom, young lord of Ruatha impressing Ruth and self-absorbed Kylara impressing Pridith. However, this could just be cuteness on the part of the author, but there's still plenty of evidence to support that dragon names rely on the rider's thoughts.

Supposed desired and un-desired traits for candidates are a general rule, not an absolute, but it is a severely rare occurrence, for example, that a queen chose a fearful or neurotic woman, as happened once in Anne's series, with Jora, who impressed Nemorth and was afraid of heights, leading to her dragon's avoidance of flight, and a severe problem with obesity.

Traits that dragons may prefer at impression: 
All that is really known is, that riders encourage the candidates not to be afraid on the hatching sands. But that might just be to reassure them in the face of the cantankerous hatchlings. It's not as easy to get noticed by a dragon when you use your peers as human shields. I believe that frightened hatclings need reassurance and are drawn to confident individuals.

Affects of impression on humans: 
  The moment of impression will always trigger emotions of sentiment to recall, even for the most hardened of riders. They tend to cope better with stress, and sympathize easier than those who have not impressed.

  In the first Passes, scientists obsessively conducted research on the affects of the bond. As time passed, though, early research could have been lost. Each Weyr kept detailed records regarding the alterations in the behavior of riders following impression. Whether Weyrs had maintained the habit into the 9th Pass is uncertain. Ultimately, no amount of records and research can take the place of personal experience, and the phenomenon will remain an enigma to all but the dragons and their riders.

Terms for an impressed pair:
  Bonded, weyrmate, soul-mate, [dragon] looks to [rider], impressed, recognized

The significance of rider sexuality:   
  Stated before, dragons search for specific genders to go with their role. Their decision is influenced by both mental and olfactory cues. As Kitti Ping Yung seems to have believed in gender roles, she assigned greens and golds to choose females. She prevented golds from being able to chew firestone so that they would not endanger themselves to fight thread as the only breeding females. (Ironically the brave Weyrwomen simply slung flamethrowers on their backs and joined the other dragons anyhow.) After the First Pass women were allowed to be taken by the weyrs less and less because holders were busy trying to marry off their daughters in a race to secure more land. 

  Interesting note from the books: K'vin, Weyrleader of Telgar at the end of the first interval, did a study of fighting tactics that revealed a statistical tendency for greens who looked to male riders to be more "volatile" and disobedient of orders in the excitement of a Fall. This does not need to be significant during role-play but is probably due to dragons sharing rider temperament, and testosterone and all that.

  Kitti Ping intended blues, browns, and bronzes to impress males. The fact that blues often impress gay or bisexual men, (implied strongly in First Fall and many other books) may be a way to "collect" impressable youths who fall into the "other" orientation/gender category. This would be so they don't accumulate as candidates left behind on the sands as a consequence of the other dragons preferring specific and more obvious gender and sexual orientations. Impressable humans are a valuable resource, especially at times when holds are more reluctant to hand over their children. It would only make sense that blues are not *always* gay or bi, but typically they are masculine, regardless of physical gender, as greens will also impress to feminine individuals regardless of gender.

  Bronzes and golds however, do not impress anything but individuals who indicate only heterosexual/cisgender cues and are physically the same sex as the dragon itself. Heterosexual pairs of men and women are more common , and therefore likely to form  attraction, or companionship; a bond that helps them as a team in running a weyr. Traits that typically come along with a male and female pair of minds are likely to ensure balanced leadership. Women tend to be nurturing, and and empathic, and males potective and adventurous. These are common traits of gold and bronze riders. They don't always get along, and don't always become weyrmates, but the balance is tried and true in the eyes of the dragons' creator: Kitti Ping.

  As a general rule, greens prefer female riders, but can impress to men, and do if they are presented no other options. Although Anne said outright that male greenriders are gay, I don't think this should mean they can't have straight relationships, particularly if this were the real world. Just because someone has a "female" mind does not mean they act overtly feminine or fail to develop crushes on women. Human preference can be a very fluid thing. Ultimately this matter is up to individual fanweyrs and roll-players.

Canon exceptions to dragonrider sexuality:

 Realistically speaking, bronzeriders can be bi. Canonically, they shouldn't have genetic or obvious mental cues that would make them homosexual. They must be attracted to women by default and be able to chose as individuals their own level of experimentation and through personal experiences find their own comfort level with the same sex. They cannot be feminine or transgendered as male-to-female. They can be sensitive. It is the same for goldriders only inverse.

Brown dragons prefer masculine cisgender individuals. There are some exceptions allowed: females with masculine brain-design. Their masculinity must reside in strong genetic and mental cues. This does not mean that a woman who impresses a brown is not allowed to adapt to a feminine lifestyle. For example: wearing a dress, having long hair. Her masculine cues are only in the brain-structure she was born with, or developed subconsciously.

Blue dragons almost prefer bisexual, homosexual, transgendered people. But they can impress straight people as well, as long as they are masculine, tomboy, or not conventionally feminine in some way.

Green dragons prefer feminine or nurturing/sensitive personalities but if they are out of options, don't really care. There are no limitations on gender or sexual orientation with green dragons.