Of course, in comparison with arma 2, lighting in arma 3 has received noticeable improvement in the alpha. Unfortunately, and this is my opinion, the overall lighting still doesn't look as good as you would expect from a game in 2013.
I noticed that in the graphics settings you can't set HDR higher than "standard", that gives me some hope, that there might be higher settings available in the final version, but i don't put too much hope in it, so i'd rather mention my concern right now.
I think lighting is one of the most important factors too make a game look more realistic, even if a game has bad textures or animations, good lighting can make a huge difference on the overall graphic impression of the game.
There is a thread over at the BI forums, that shows the impact lighting has on a scene quite well (just read the first post): http://forums.bistudio.com/showthread.php?127228-Improving-the-Light-Engine-What-and-How
At the time Arma 2 came out, i found its graphics to be impressing enough, with arma 3, even it might look better at some things than arma 2, this is not the case. Improvements are too small to call this a game, which has satisfying graphics for 2013.
Other engines, like the frostbite engine, use middleware like this for lighting: http://www.youtube.com/watch?v=JZO1LF305UE
I read particularly this solution - for example, scales pretty good on multicore, i don't understand, why such a solution can't be integrated into arma. If it takes too much resources, alright, make an ultra setting for lighting, that only people with high end machines can use.
Then, in one or two years, graphic cards will be evolved enough, so that everyone can play on ultra without having a high end machine.
I understand that Arma is an unbelievably huge open world game, that takes up quite some resources and i appreciate all the effort. But seriously, i think after 4 years, since arma 2 was realeased, we could expect this game too look significantly better than this, starting with the lighting.