IntelligenceIf an alien species is going to be a meaningful part of diplomatic negotiations (successful or otherwise), it's going to have to be at least as smart as humans. If it's going to be the one who seeks us out, then unless we already have FTL technology by the time of whenever the story is set, they're going to default to smarter than us. Why? Because the nearest galaxy is maybe a dozen light years away, and your alien species will need to have determined that there's something worth checking out on our insignificant little planet, or else they wouldn't have bothered.
That's not to say that individual aliens within your species can't be idiots. You get those all over.
AppearanceAs indicated above, this can range from really really human to really really not. Many early aliens in visual media looked human simply because there was no such thing as ILM. In the Star Trek universe, most of the sympathetic aliens resemble humans with accessories because the Progenitor seeded a butt-ton of world with genetic material, and let evolution do the rest. Star Wars, being set in a space opera universe with a bigger budget, has a wide variety of aliens, made even more diverse with the addition of motion-capture and fully CGI sharacters. In written media, of course, you have a lot more freedom to make your aliens look like whatever the heck you want, but be cautious with aliens that are obviously not humans with weird makeup.
Non-humanlike aliens can even resemble familiar Earth species, like cat people (a favorite, for some reason), lizard people, or even insects. In general, the less mammalian they look, the harder it is for we humans to relate to them. Give them weird senses and a corresponding lack of familiar anatomy, and the average first contact team is going to be cautious, if not initially afraid of them. Once you start getting into the more mind-bending aliens like sapient colors (I have seen this played for laughs and for horror), it gets harder and harder to comprehend them, until you get the sort of things that should not logically follow the same laws pf physics, let alone biology, that we do.