My two cents...

[liebowa]

There is nothing earth-shattering here - just the basics (that I'm sure have been echoed again and again):

1.) REAL overcast.
2.) Better ATC/AI integration - I KNOW this must be VERY difficult, but It's SO irritated after a multi hour flight to be confronted with these nincompoops on approach and final to a busy airport. I'm glad it's there - I just wish it were "better".
3.) Better Save Flight implementation. There has been a hundred times that I would have liked to save the flight to pick up at a later time - but just shut everything down instead because I knew that when I reloaded the flight the rest of the environment wouldn't have been where I had left off.
4.) Better weather transition effect smoothing. Last night I was flying an approach into KIAD and the clouds/environment just kept shifting back and forth between bright/dark, rain/no-rain, etc. It can make for a shoddy, unrealistic and depressing flight having such sudden and harsh graphical changes.

Other than that - I'm extremely happy with this simulator...

[FlyingColin]

I totally agree with the above-mentioned posts..
I just want to reaffirm the idea that the simulator has a lot of promise as it is right now, but that making modifications to improve it will help. In my mind, as a real-life pilot and working towards my commercial rating, I can say that the two most important things for me in the simulator is:
1) ATC
2) Weather

It would be nice to not have Trump real estate and Sheridan Inns at the edge of the runup pad, but aside from add-on scenery that can fix a lot of this, most of my flying is only looking at the instrument panel to keep proficiency and I almost ALWAYS fly in the clouds either way. The scenery isn't quite what it should be for enjoying as much VFR as I can in the real-world, but at altitude it looks much better.

In regards to ATC...having the ability to declare in-flight emergencies and have them take priority to those would be great! Squawking 7700 might set off an emergency bell and ATC would query you as to the sqauwking of this code. Another aspect of ATC that a lot of ppl have mentioned is the integration in DP's and STAR's into departures/arrivals. Less vectoring and handing off/onto the same two frequencies multiple times a minute. Perhaps the addition of some automatically updatable IAP's and such...
Improving the flight planner so you can more easily type in fixes, NAVAID's, RNAV stuff, etc. By having an interactive flight planning area you will find it easier to integrate flight plans into the system and VATSIM, etc.
And yes, of course, the improved AI characteristics - definitely! Getting off the runway more realistically, taxi speeds, new airliners, and mixing it up in terms of the models represented.

As for weather...it's already been mentioned several times so I'll just sum...but basically having weather "seaming" together between stations would be great. Not flying along in the clouds and suddenly it's clear and a million and you look straight back expecting to see that nasty cloud you got out of, and nothing's there. Having the weather look and feel right flying is very important, I think. Having a 12 ft. snowdrift on some tower is awesome, but not key to the flight simulation aspect to it.
Making sure that the wind shifts are realistic without over doing it on the gyros.
Another important weather aspect is fronts, airmasses, how/where they collide, etc. I don't think tornadoes are an integral part of a simulator, but definitely huge storms should be. Half the sky can be blue while the other half is extremely dark...
Rain showers, rain and snow that vary with intensity are important as well as perhaps a visual of ice-build up. Frost, etc.

Again...the fact that we are trying to simulate the world better and better is going to be harder and harder, I know. But I'm very happy with everything that's been done so far!