These are changes that, to my knowledge, weren't welcome:
- Homeworld Remastered lacked ballistic model, as in, instead of simulating projectiles flying from ships to their targets, it just used RNG to determine hits or missed. This was fixed in version 2.0 available on GOG
- Formations in Homeworld Remastered used to not work properly, which was again fixed in version 2.0 availible on GOG
- Tactical settings for your craft was lacking in Remaster, it's now present in 2.0
- Fighters in the original Homeworld had a fuel gauge - IIRC your fighters needed to routinely refuel. It's a feature which sounds extremely cool on paper but ends up being a huge hassle and got removed in Cataclysm and subsequently Homeworld 2 and Remaster.
- Difficulty scaling was milder in the original. Contrary to popular belief, both Homeworld Classic and Homeworld Remaster get more difficult as your fleet increases - thing is, in Remastered version, this effect is a lot more visible and subsequently, the game is more difficult. Which I believe to be a good thing, because frankly, if you played the original decently, about 1/3 of the campaign was absolutely primitive in its difficulty.
- Combat is faster. Ships generally take less punishment before they go down. Again, this is a change I personally welcome as I always found tiny fighters taking a ton of punishment before going down extremely weird and some combat in the original had a tendency to drag.
- There are also other changes - voice-acting got re-recorded with different actors, but VA in HW1 and 2 was not really the strong suite anyway (it was a lot better in Cataclysm, alas...) and music got spruced up. And then of course all the changed cosmetics.
All in all, if you've never played Homeworld before and don't have nostalgia goggles, it's very likely you'll enjoy Remastered version a lot more than the Classic version, if nothing else purely due to spruced up visuals and quality of life features it introduces. As I said tho - there's no reason to take anyone's word for it. First two missions are about 40 minutes, just try them in both versions, preferably Classic first otherwise your judgement will be clouded by sudden loss of quality of life features. And there you go. Either way it's probably the best RTS I've ever played so enjoy!