Personally, already played beta more than I've played first game. Small little changes here and there made it more enjoyable.
- Like ability to break UFO walls make ground mission feel much less grindy as exactly the same UFO can now be breached in a different way
- New way of management of Scientist/Engineers feels more engaging, previous version was more of "how much money I spend on these guys" type of thing
- I'm loving new sounds, weapons sound like they have a good punch behind them
- reworked aliens and characters look pretty good in 3D and it's easier to read "specials"
There are other things but this is just what jumped immediately to mind.