as others have said your exposures are too short to pick up the dust/nebulosity around the primary stars. you could try going to 6s or up to 15s without trailing. try that with iso1600 and see what happens. Watch your histogram.
edit: as far as exposure goes, if you're only doing a single non "hdr" image, meaning one set of subs with the same exposures, you're trying to balance the dynamic range of your sensor. for instance, last night i imaged this for the first time. i did a trial exposure which picked up the nebulosity well, but looking at the stars and histogram it felt a bit hot. i did another trial exposure this time being quite a bit shorter to see what the highlights in the stars looked like vs nebulosity. there was no bloat in the stars but there was no nebulosity. so i chose an exposure between the two but making sure i got the nebulosity showing up. trying to bring that faint nebulosity out in post processing would probably introduce a lot of noise. so its a bit of give and take between your brightest highlights and faintests regions trying to balance which is most important to retain detail in. of course if the dynamic range is too much for your camera then you'd have to have two sets of exposure to retain details in the most important highlights and shadows.
as far as color, the primary stars to my eye seem too neon blue. compositionally it seems off balance by including mars way at the bottom at the edge of the frame. it almost feels like an after thought. if wanting to include mars with M45 try including a much wider field of view and that would help balance the composition, at least to my way of thinking.
. i hope this helps! the good thing with digital is you can just keep trying different settings!
I use APP and PS for post processing and find it very intuitive to use. but i do come from a landscape bg and have a good feel of PP.
