Multiplayer used to work but never tend to try it out since I use third party addon apps, but I hear it's mostly full of folk being silly, there are communities online that can give you access to their slightly more mature servers if realistic flying is your thing.
Most of the difference is for steam edition to the old boxed edition there seems to be a pleasantly surprised at how many more fps your getting! It is noticeable too. UK scenery based on real-world aerial photography, detailed 3d airports for the UK, both civil and military, finally, the pride of place, pmdg provide a fresh update, steam compatible of their procedural Boeing 737 800/900/600/700 ngx aircraft Addon scenery and aircraft dramatically enhance the basic flight simulator, pushing the limits nearer what current computer hardware is capable of. Secondly Dovetail have tweaked things slightly, DX10 preview actually increases framerates now rather than dropping them, the clouds have been replaced with better ones, the sky is better, they seemed to of ruined the Sun textures and now it appears very small in the sky however if you have a multi-monitor setup you will be using a much closer FOV or zoom setting so the sun might seem better then, also there's "Nicks Sun MOD" which is a good replacement of the default fsx sun. So what's different from the retail (Microsoft) version? Well, first thing to notice is you get ALL the planes that you'd get with the Gold and Deluxe versions of FSX, which is nice, oh and the extra cities. (by the way they have a new 64bit revamp of fsx called FSW or Flight Sim World, if you're lucky like me and bought Flight School which is also based on fsx from Dovetail you will be given FSW for free!, so far it's good, but needs a lot of work as time/season/weather are all locked to specific themes, we want real-world weather again and hopefully they will bring that back) As suspected due to the huge amount of power it required back in its day of release, it has certainly stood the test of time, and this version was DOVETAIL GAMES until Microsoft bought the rights back in 2019.
Not a lot to say really, If your buying/ looking at this your likely to already have the boxed retail version from 2006, this is a download version of the famous Microsoft DVD rom flight simulator x.