I love classic Traveller, and some of my best gaming experiences have been with it. It's different from a lot of other (modern) games, though.
1. Character creation, even removing death during chargen, yields wildly variable characters. Many modern players don't like that. I've made my own rules for normalizing it, but as written there's a lot of variability. At the same time, you can play a whole campaign with a character with Vacc-1 and no other skills. (From the sounds of things you're only using books 1-3. Adding supp 4 is fine, but once you gets books 4-7 mixed in, those characters are far more capable.)
2. There's very little character development. No zero-to-hero. Once you're adventuring, you can become more and more financially stable, sometimes, but really that's it. It's about surviving in the universe, not about levelling up.
3. Whether you're using the Third Imperium or you have just rolled up your own subsector, the world is a sandbox. You can present missions for the players (mercenary tickets, or whatever), but often players just want to explore and player whim might take the characters tens of lightyears away from an adventure you have prepared.
4. Combat is deadly. There's tons of granularity in the system, as weapons and armour intersect with one another, but they all are going to kill you.
5. The skill system inadequate. Players will quickly learn that they don't want to roll for things, and make choices to avoid extremes where failure is likely. It's what most of us do in life. Letting characters explain why they might have some basic knowledge even if it's not on their sheet is part of the fun, and helps fill out their background. But as ref, you need to allow them to do that.
6. Despite all of this, it's an awesome game. The world created by the system is rich and open to endless adventure, and boogieing around in a little spaceship with your friends is the future I wish for us all.
