Lysander said:BTW, surprised some here think the Kafers aren't interesting...our group imported them into our Traveller game some 20 years ago. We love the Kafers....dead preferably.
One of the reasons I loved the 2300AD over OTU were the aliens. Much more interesting and plausible then Cat Men, Dog Men, Space Elves, Space Vikings, Space Cows etc. In my opinion only the Hivers and Droyne are on equal footing to the original, interesting alien cultures in 2300AD.
[EDIT for counter-arguments, sorry another list

Kafer: Violent bipeds makes them comparible to "Space Orcs" yes. Otherwise, no. The most unique quality was (spoiler alert) their violence induced intelligent gain.
Sung: Do have a similar appearance to Droyne (birdlike biped with wings). Most interesting quaility I can think of is their culture which requires more advanced civilizations to share their technology and wisdom while those of a lesser culture/tech to be submissive.
Xiang: Multi-legged makes them "Space Bugs". Interesting historical background of being slaves to the Sung. Difficult to be recognized as intelligent (hard to communicate with) while having artistic talents makes them unlike "Space Bugs" which instead would have given them a Queen, Nest, Workers, etc.
Eber: I can't think of a comparison for their appearance. Their culture is also strange and enigmatic. On second thought, are they "Space Ents"? "Space Indians"?
Pentapods: I guess their multi-limbs (like Hivers) makes them "Space Squids". But, also like the Hivers they have an unique and interesting culture - biotech experts, living technology including spaceships, hive mind.