This is some, not total, conincidence between the quality of Fairfax County and northern Virginia neighborhoods and the amount of transit provided, though often other factors are at play. Look at this map
https://www.wmata.com/pdfs/bus/Virginia_System_Map.pdf showing where bus routes run, whether rush hour only, weekday midday and evening also, and or if service is six or seven days a week (you will need to look at the individual bus schedules online to see how late or frequent the service runs). Areas such as Great Falls, Clifton, and Virginia Run in Centreville have no bus service, short of driving a distance to a park and ride lot, and that is unlikely to change in the forseeable future. On the other side, areas such as Lincolnia, central Annandale, and Culmore have frequent service until after midnight. Even center Chantilly and Centreville have weekday bus service until around 10pm, though limited weekend bus service may be coming to these area later in county FY16 depending on public feedback (its already budgeted). Some areas have premium priced commuter bus service which can be good for an area, such as Fairfax Connector 599 from Wiehle-Reston.
From observation and even 1st hand past experience, increase transit to an area, demand is going to increase for housing in that area. From there, real estate and rental prices are going to increase an extent (maybe not overnight) to maintain neighborhood quality or if prices increase very little or not at all, the areas stagnate - not always, but more often than not. My faith leads me to care for the poor and to respect diversity (currently have and have had friends of other races and income levels, work at a local food pantry) - not Cathlolic like the Pope before Congress last week, but along those lines - however, 1) diversity which creates a health or safety situation, you cannot communicate with your neighbors in English in a 911 neighborhood situation (been there, done that) or the neighborhood environment health situation nearly sends you to the emergency room is not good, or 2) where the diverse behavior is just plain bad or immoral, as I have met bad white people, good Indian people, etc., just to say I am white and upset too with the bad effects of diversity, loitering and not using a public indoor bathroom or playing loud music to annoy your neighbors for example (not care if the people are purple with orange dots), though I lived in areas with seven day a week neighborhood transit in the past and apartment experiences there not good in the end, am now where only commuter transit and very limited offpeak weekDAY service, or 3) where the diversity is so much it suppresses your consitutional freedom of religion (a general county issue to at least a limited extent and more so in FCPS, but a greater USA country issue). Am I going to be in Fairfax County in 20 years, a ?, but have over the years moved from eastern, then central, to western Fairfax County.
Other factors at play in neighborhood quality, what stores if any are adjacent to a neighborhood. Shoppers Food Warehouse, Walmart, Dollar Tree, and the like - neighborhood not so good or prices high. On the other side, Wegmanns, Harris Teeter, etc. Just generalities, there are exceptions. Also Fairfax County TV cable systemas cater to a lot of languages, not so much so in counties further out.