There's a flightsim community on Reddit and they are of course talking about this extensively, just like everyone else in the community is.
Recently, someone posted a request for thoughts/concerns/hopes for the future in regard to Dovetail's acquisition and the forthcoming state of flight simulation in general.
Here are the thoughts I posted....
Dovetail's purchase of the FSX license and move to put FSX on Steam will hopefully bring with it wider public awareness of FSX and Flight Simulation. Of course, there are some who are dreading the influx of those who may not be familiar with realistic simulation programs, while others are eager to embrace the newcomers. The flight simulation community is extremely protective of the fidelity of their "simulation" environment, and cringes at any notion of an arcade/casual experience, but at the same time, the majority are welcoming to newcomers who express a desire to learn about the complexities of flight and challenge themselves in a program not focused on high scores or killing the most creatures.
The third party environment for simulators such as FSX and the multitude of train sims out there may catch some new users off guard. Writing specifically in regard to FSX, the addon market is full of sometimes expensive products for virtually every aspect of the program, which may concern those who are used to free "DLC" or addons at a minimal cost. Realistically speaking though, if Dovetail moves to create further addons under their own name for FSX, it really will be no different than the situation we have now, with the innumerable addon companies out there charging anywhere between $30 and $80 on average for a high quality aircraft, scenery, or environmental products.
In general, and related strictly to Dovetail's adoption of FSX, I really don't see a drawback unless they modify the license to limit or prohibit third party content, which I see as an unrealistic option that would cause significant negative press if they were to enact that policy.
The real unknown is what will happen in the case of Dovetail's new simulator, at this time alleged for a 2015 release. There are so many options that they could take, and I believe a current worry is that they will bring forward a product which requires dozens/hundreds of microtransactions in order to experience fully. An additional worry is the departure of the "simulator" spirit to that of a more "arcade" style.
I believe that their future flight simulation product(s) need to retain the "hardcore simulation" aspects that the existing community is accustomed to, while also supporting extensive third party development, but an additional critical factor for wider success also rests in the ability to make their program accessible to the general public who may be seeking a more casual/arcade experience. It's possible to have a simulator and a casual arcade experience in one program. Enabling the ability of the end-user to highly customize/modify their experience to suit their needs and desires is paramount for their success and wider public acceptance of their flight program.
SeanTK
EDIT: Exclusive comments for NZFF....

As an FAA licensed pilot, I value the fact that I have a very realistic simulator at my disposal which I can use on my own time, in the comfort of my own home, at no hourly cost to me. FSX is fantastic when it comes to simulating IFR flight, even without the plethora of addons out there. The ability to consistently practice and refresh oneself on the intricacies of various routes, approaches, and the like increases the value of FSX exponentially to me.
During my private license (non-instrument) training, I used the simulator (back then it was FS2004) to practice routes that I would fly for real in the next day or so. Even with the default simulator scenery, when it came time to do the real flight, I could associate things I saw in the sim during my real trip, thus greatly aiding my preparedness and situational awareness. As I started learning the IFR system, FS2004 and FSX became even more valuable as I could practice navigation (departures, enroute navigation, holds, approaches) extensively in an accurate environment on par with the expensive hourly simulator sessions at the flight school. I could use real charts and correlate them in FSX to realistic headings and frequencies, and fly a simulated Cessna 172 at realistic altitudes and airspeeds in convincing weather. Then, I would do the flight in the real world and remain miles ahead of the plane in terms of knowing what to expect while having excellent situational awareness.
Microsoft Flight Simulator is a safety enhancement to my real flying. The point I'm gradually getting at here is that I sincerely hope that future flight simulation products from Dovetail or whoever maintain the ability to conduct realistic operations and truly "simulate" flight and all that goes into it. It is of course important to expose the product to a wider, casual audience, but I hope that the program maintains the option to turn off the "arcade" mode and engage with the program in the way that the community is already accustomed to doing.