The difference is ASTOUNDING. 2.3.1, broken reaction fire, eh graphics, 3D worldview got a nice facelift from last time I played (can't remember when exactly), tons more maps from when I last played which is really nice
ok. ok. I played that for the past couple days, then decided I'd stick 2.4 somewhere else so it's newness and dev-state-ness wouldn't wreck my carefully pieced together saves
holy crap, never going back. The particle effects are **GORGEOUS**. Seeing a flurry of tracers from the machinegun is seriously like a religious experience. Explosions are EXPLOSIONS. The aliens screeching on hit and death is an amazing touch. Hits actually register with bloodsplatter or noise or accurate tracer impacts or all three!
Also reaction fire seems to WORK. I set up a bunch of snipers covering the advance to a Harvester, certain they wouldn't do anything as aliens strolled among my forces with Kerrblades... but no. One Taman showed his head for just an instant, then a bloodspider moved a little further than the Taman had (this was my first mission, I was a little O_o at a bloodspider already), and the two shots that rang out seemed aimed at the Spider but one hit and killed the Taman. Excellent.
And an Assault Rifle that can autofire!! Finally. These changes seem amazing! I literally almost melted like so much Inferno'd alien when I saw all the changes. And the alternate HUD isn't completely disorienting either (though the arrows for weapon hands is a poor touch)
A few notes: The HUD seems rather set on each soldier, and changing to another one has a delay when displaying the second soldier's stats, this is somewhat disorienting. Generally seems to be minor issues with the alternate HUD when switching displayed soldiers, ie if the initial selected is crouching and you switch by any means to one not crouching, the image updates the skin but not the pose. Also the crouch/notcrouch buttons don't seem to update correctly on the alterHUD, when swapped from crouching to not crouching as noted above the "Crouch" button was still depressed, selected, highlighted (with all those gorgeous graphics) but when I thought "Oh I'll just click Stand" the second previously standing soldier crouched, but the Stand button was highlighted. Disorienting but likely a minor fix.
Seriously. The difference is night and day. Don't even bother 2.3.1, go straight for 2.4!!!!! (I'm only in for a couple missions so far so can't comment on stability but it's an alpha/beta game anyway so)