Terrain is plenty destructible.
I played 150 hours and I still haven't finished it ( mostly due to save breaking resets) and I can assure you that it is good, and filled with a lot of good old X-Com spirit.
ground combat is definitely more varied than in the original game. You have more kinds of grenades ( flashbangs to suppress* enemies, stun grenades to, well, stun them, and another kind), there are combat shields that are able to take an hit or 2 ( frontally. mostly used to breach UFOs) but most importantly there is cover. A lot of cover, which is perhaps the core tactical element. You need to use cover well to advance safely.
Aliens may not be the smartest around, but they use cover well, and can find you. Occasionally, they even use grenades, but for whatever reason that happens rarely. Aliens often have some special ability. Some of them see farther, some regenerate, some see through smoke, some are robots, some are powerful psionic,etc. ( a few other interesting abilities, but I am leaving those for you to discover)
There are a few changes to the geoscape game.
Economy is now based mostly on funding from nations, later in the game complemented by the multitude of ground missions. But make no mistakes, losing funding nations will hurt. Manufacturing is no longer profitable.
Air combat is less abstract than it was in the original game. Here you control your interceptors on a tactical map, and need to evade enemy missiles, try to flank the enemy craft, gun them down with your cannon or blast them with your missiles. There are 2 kinds of interceptor for most of the game: a light one with cannons and light missiles and an heavy one without cannon and with the ability to mount both light and heavy missiles. This mode has an autoresolve.
Crash sites no longer give a reputation increase when completing them and if you want, you can just airstrike the site, and get a small reward. However, doing so will net you no alien materials and much less money. It is also veryquick, cheap and carries no risk. Terror sites however carry an hefty penalty if you don't show up.
And a few other things...there are plenty of utility improvements and obviously graphical improvements. Game is set in cold war, world is no longer a sphere but a map , tech tree is different, no human psionics. etc.
and of course the most dreaded enemy of old is also in the game, and this time you can't shoot while flying