I've played 2.5dev now for over a year (game time of course) and thought I'd add my two cents:
Overall the game is still great. Fighting the aliens is more difficult than I remember from 2.3/4, but generally it's good to keep it challenging. Re op - flashbangs are definately working in my game. I also get RF for my own guys, though rarely (no prob for me though), and in comparison to that alien RF seems a bit over the top...
Other than that:
1. It's cool to have smoke grenades to hide yourself, but when hiding "in the clouds" becomes the primary thing to do in some maps it is imo less fun. Also firing out of the smoke screen just to avoid getting killed feels a bit like cheating, but otoh with so deadly aliens it remains often your best choice.
The above seems to be related partly to starting positions on certain maps - sometimes your tough elite guys prefer to approach the combat zone from the worst thinkable tactical position, which means you throw smoke grenades around for some time until finally finding cover.
I never tried the map editor - can you actually modify starting positions there?
2. I have no prob with the Aliens having superior capability for reaction fire, but when they hit (nearly) everything 20-30 squares away, but otoh do nothing when you crouch or run directly in front of them it feels strange.
3. It seems particle rifles and needler guns are the most advanced weapons? If so I'm wondering whether they're coming too early or if research is too fast generally. As said, I'm in the game for over a year now and for a good deal of time the aliens only use those two basic categories of weapons and nothing else anymore.
Maybe alien teams could still use a bit variety in equipment in later battles, maybe depending on their role or UFO to make things more interesting.
Is it actually possible to mod which (of the available) weapons aliens use in specific missions and from a certain point in time?
4. Power armour slows you down a tad? I thought I get one of those nifty power suits that amplify human power
Also, it seems to do worse vs. later alien weaponry (said particle thingies). Is this intended?
5. Sometimes alien behaviour is strange - on some maps they are super-aggressive, on others more passive. This wouldn't be a prob in itself, but sometimes it's contrary to the specific type of mission you play. For example in base attacks only some of them enter my base, but the bulk stands around outside doing nothing until I hunt them down. Otoh I'd expect them to be more on the defense when their UFO just got knocked out of the sky by PHLX interceptors.
6. There are some nice maps I have not seen before, but overall there seems to be less variety than I remember. I skimmed through other threads, read some maps have probs now, so maybe this is only a temporary issue.
7. When firing multi shots using beam weapons like lasers and PBs the beams sometimes start way besides the weapon (probably a known prob)
8. Imo, aliens should die more often instead of get stunned purely from wounds. At least in my campaign I barely had to make serious efforts to capture aliens alive because many simply got stunned during combat (maybe luck on my part though).
Overall still a great game, far from "sucking completely"