Obviously the best thing to do is use the ED forum link to go vote, but here’s my rationale on a Sunday morning (just about, thank you DST)
4th generation, multirole fighters
I, like everyone else I think, is keen to see a F/A-18 of any form, but I’m in the curious spot of being in a backlog of aircraft to go learn. I can wait for this a bit. I also have more of an itch for ground pounding, and the A10C/Su25 helps me out here already. If I was more of a F15/Su27 guy then this would probably have got my vote.
Dynamic campaign system
I don’t think anyone knows what this really means, in that it’s become a catch-all phrase for ‘whatever you can dream you want’. There are branching, story campaigns already, so while I can see the appeal of the ‘strategy map’ view, with resource constraints, battle bubble etc, it’s not something I personally would vote #1. If it wins (and I predict it will) then that will allow the conversation to move on to more a ‘So what exactly is this thing?’ stage.
New combat theater maps
I cheat a little here and assume NTTR is coming anyway. The question I asked myself was, ‘Do I need Straits of Hormuz as soon as possible, i.e. my #1’. I think NTTR will keep me busy for a bit.
Improved multiplayer with dedicated server and online performance tracking
I don’t MP enough in DCS, but am trying to get into it more. I get stuck on not committing to the time it takes to do it properly. I appreciate it’s an important part of DCS though.
New and improved air traffic control system
I really like this one, in that when I did play SP, having a realistic ATC state machine would be something I’d love. I actually voted for this one, the least sexy of them all! 
New and improved effects and audio and visuals
Would be nice, but I like the improvements in 1.5 already, and spend a lot of time staring at the cockpit/MFDs rather than outside.
More content in the form of missions and campaigns
I’m still behind on completing what is there!
Other. Please list in discussion
I’d like dev dollars spent on things like VR support, maybe in-engine cut-scene support for missions (more story telling) and things like better tutorials and new player support helpers (better Xbox One controller default bindings, a better controller bindings library centralized etc). Stuff that makes it easy for new players to get into DCS.
New helicopters, like attack helos
Apache would probably be my #1 here, but narrowly shaded by the ATC bits.
Older era aricraft from WWII to Vietnam
Would be great, but I’m not short of modules/content to learn. I have a preference for Vietnam era rather than WWII as I’ve spent a lot of time in that area already. I guess I just see DCS as more a ‘systems fidelity’ thing of which the war birds are simpler to fiddle with and the 60/70s were a systems skills test (re:MiG-21).
Good stuff @wagmatt - interesting to see the results. Obviously the results will have exactly 10% of the vote each