>>2130469
There's a lot of small things like more weapons being useful, research tree being more complex although that came with a bug in the original version with the tasoth commander bricking your savefile but that's all fixed if you play openxcom.
The game is harder yes but it's also more fair due to better game design. In vanilla you had heavy plasma which has potential to one shot literally everything short of sectopod, player units can never reliably survive a heavy plasma hit, ever.
Introducing Ion armor, the thing that sets apart these two games the most. In vanilla ufo defense power suit would have 100 front armor and like 90 sides 80 rear, it doesn't matter though because you will die to 1 hit from heavy plasma regardless where it hits.
In TFTD Ion armor has 132 front armor 70 sides and 100 rear, there's an enormous difference between getting hit in the front and sides. 132 also means that getting hit in the front plate means that your soldier will survive a hit from a sonic cannon most of the time. So your soldier survival is much more based upon good positioning rather than pure luck.
If you just look at my stats between two games, I lost fewer soldiers in TFTD despite it being a harder game because the skill ceiling is just much higher.