Generally, I prefer to talk about things I like more than things I don't. 
I've played Traveller since 1979 in various editions. I never actually played T4/T5, but I've read them. And they all generally have little nuggets that I like to keep. Overall, my favorite edition is MgT2e, but there's stuff from elsewhere that I like.
Classic Traveller
1) Unrefined Fuel actually matters. In BK2, using unrefined fuel wasn't just a misjump risk, but also increased the chance of maintenance problems with the engines in general. Even with BK5 and the introduction of shipboard purification, they were much larger than modern versions at the usual TL 10-12 that tramp traders were.
2) Medals and Bonuses in Character Creation. Advanced characters (Bk 4/5/7) could get medals (military) or bonuses (merchants) in addition to skills. T5 adds similar options for other character types.
3) Lasers are really scary if you don't have Reflec/Ablat/Battledress. Mid range armors like mesh/cloth/etc basically were somewhere between worthless and actively bad against laser weaponry. It's why lasers are so heavily restricted in the Law Level tables despite being less effective than automatic weapons in current rules. Haven't actually made any rules I like for this, though. Just wish it was true
MegaTraveller
There's probably stuff I use that originated in MegaTraveller, but I lost that rulebook to a flood long ago and I can't recall much about the rules specifically.
New Era
4) More skills for Terms 1-4 than Terms 5+. Not a fan of term limits or the very harsh aging of Traveller's rules, but I do like short timers being more equal to long timers. And also going straight into a career being more equal to going to school.
5) Lots more types of schooling Law School, Flight School, Technical School, Grad School, etc
6) Ship Shares: New Era's ship shares are, imho, much more interesting than other editions. Everyone in the party pools the ship shares they got. Can buy extra ship shares for Cr50k. Roll on the relevant chart to get a class of ship and how messed up it was. PCs take the resulting negatives and distribute the points between "ship is worn down", "we have a mortgage", and "modifiers on the ship size table". So players have a lot of agency with what kinds of challenges they have with their ship.
7) Heplar: Really want to love reaction drives instead of space magic woo-woo drives, but even super efficient reaction engines are still tyrrany of the rocket equation in actual play. I just have never had players interested in messing around with fuel usage and thrust & drift mechanics. Still like HePlar, though
T4
8) The psionics institute stuff in case you want to see how psionics might work in a non psi dominated society where they are generally legal.
9) Body Pistols are actually disguised guns, whether it's your iPad stylus or a belt buckle or a replacement finger bone. Not just a future version of the "plastic glock" scare of the 80s.
T5
T5's rules are just too weird for me to actually use them. But there's a ton of interesting stuff in the books.
MgT2300
10) Radiators! I just like ships having to have actual solutions to heat. Though since I am reliant on other peoples' art skills, I usually assume all Traveller ships have the advanced hull panel radiators, not the big sticking out like wings panels
11) Sensors! Related to radiators, Mongoose 2300 just does ship sensors better than other versions, imho.
12) Planetary Adaptation Syndrome: Not the specific rolls, as I think they are too harsh for frequent travelles like the PCs. But I like the idea that off world travellers are rare because most people get super Jetlag trying to deal with the different gravity/sky color/sunlight/air pressure/day-night cycles/microbes of other worlds.
13) Surface to Orbit matters! I like that its unlikely a ship will actually land on a planet. I like shuttles and beanstalks and space catapults and little tramp traders being airframes or lifting bodies so they can go directly to the downport (or middle of nowhere) for trade. And I like runaway landings for my fat traders
MgT1e
14) I liked that Mongoose 1e had bigger career events tables. D66 options instead of 2d6 reduces chances of duplicates, either on one character or between different characters of the same career.
Clement Sector
Cepheus isn't technically traveller, but whatever I like the longer lifespans in this version. I just don't think that the cliff falls out from folks at 34. Sure, I'm not as spry today as I was 25 years ago, but it is nothing like the calamitous effects of aging rolls in standard Traveller. And, honestly, there's enough skills and subskills that an extra several ranks being possible aren't going to wreck anything.
Anyway, just felt like calling out some things from other editions that I like.
(Didn't talk about GURPS/HERO/T20. I have those too, but I consider them to be Charted Space settings for those rules, not Traveller rules per se. All the good rules in them are like setting background stuff, not things that affect gameplay. Imho)
I've played Traveller since 1979 in various editions. I never actually played T4/T5, but I've read them. And they all generally have little nuggets that I like to keep. Overall, my favorite edition is MgT2e, but there's stuff from elsewhere that I like.Classic Traveller
1) Unrefined Fuel actually matters. In BK2, using unrefined fuel wasn't just a misjump risk, but also increased the chance of maintenance problems with the engines in general. Even with BK5 and the introduction of shipboard purification, they were much larger than modern versions at the usual TL 10-12 that tramp traders were.
2) Medals and Bonuses in Character Creation. Advanced characters (Bk 4/5/7) could get medals (military) or bonuses (merchants) in addition to skills. T5 adds similar options for other character types.
3) Lasers are really scary if you don't have Reflec/Ablat/Battledress. Mid range armors like mesh/cloth/etc basically were somewhere between worthless and actively bad against laser weaponry. It's why lasers are so heavily restricted in the Law Level tables despite being less effective than automatic weapons in current rules. Haven't actually made any rules I like for this, though. Just wish it was true
MegaTraveller
There's probably stuff I use that originated in MegaTraveller, but I lost that rulebook to a flood long ago and I can't recall much about the rules specifically.
New Era
4) More skills for Terms 1-4 than Terms 5+. Not a fan of term limits or the very harsh aging of Traveller's rules, but I do like short timers being more equal to long timers. And also going straight into a career being more equal to going to school.
5) Lots more types of schooling
Law School, Flight School, Technical School, Grad School, etc6) Ship Shares: New Era's ship shares are, imho, much more interesting than other editions. Everyone in the party pools the ship shares they got. Can buy extra ship shares for Cr50k. Roll on the relevant chart to get a class of ship and how messed up it was. PCs take the resulting negatives and distribute the points between "ship is worn down", "we have a mortgage", and "modifiers on the ship size table". So players have a lot of agency with what kinds of challenges they have with their ship.
7) Heplar: Really want to love reaction drives instead of space magic woo-woo drives, but even super efficient reaction engines are still tyrrany of the rocket equation in actual play.
I just have never had players interested in messing around with fuel usage and thrust & drift mechanics. Still like HePlar, though T4
8) The psionics institute stuff in case you want to see how psionics might work in a non psi dominated society where they are generally legal.
9) Body Pistols are actually disguised guns, whether it's your iPad stylus or a belt buckle or a replacement finger bone. Not just a future version of the "plastic glock" scare of the 80s.
T5
T5's rules are just too weird for me to actually use them. But there's a ton of interesting stuff in the books.
MgT2300
10) Radiators! I just like ships having to have actual solutions to heat. Though since I am reliant on other peoples' art skills, I usually assume all Traveller ships have the advanced hull panel radiators, not the big sticking out like wings panels
11) Sensors! Related to radiators, Mongoose 2300 just does ship sensors better than other versions, imho.
12) Planetary Adaptation Syndrome: Not the specific rolls, as I think they are too harsh for frequent travelles like the PCs. But I like the idea that off world travellers are rare because most people get super Jetlag trying to deal with the different gravity/sky color/sunlight/air pressure/day-night cycles/microbes of other worlds.
13) Surface to Orbit matters! I like that its unlikely a ship will actually land on a planet. I like shuttles and beanstalks and space catapults and little tramp traders being airframes or lifting bodies so they can go directly to the downport (or middle of nowhere) for trade. And I like runaway landings for my fat traders
MgT1e
14) I liked that Mongoose 1e had bigger career events tables. D66 options instead of 2d6 reduces chances of duplicates, either on one character or between different characters of the same career.
Clement Sector
Cepheus isn't technically traveller, but whatever
I like the longer lifespans in this version. I just don't think that the cliff falls out from folks at 34. Sure, I'm not as spry today as I was 25 years ago, but it is nothing like the calamitous effects of aging rolls in standard Traveller. And, honestly, there's enough skills and subskills that an extra several ranks being possible aren't going to wreck anything.Anyway, just felt like calling out some things from other editions that I like.
(Didn't talk about GURPS/HERO/T20. I have those too, but I consider them to be Charted Space settings for those rules, not Traveller rules per se. All the good rules in them are like setting background stuff, not things that affect gameplay. Imho)
