This is getting more and more relvent since solid competition is starting to crop up everywhere. Gitlab, Bitbucket, GOGs, Gitolite, Gerrit. Not apples to apples of course, but Enterprise has a lot of decisions when it comes to git control.
Even the things that could be considered Github "lockins" aren't really so: Google's git-appraise[0] could make pull-requests portable, Github's wikis are portable by their very nature[1], software like git-issues[2] could make issues portable. Not in the immediate, but decentralize-everything is a rising sentiment.
The only real lockins I see are Github's work-flow and social community. If either become toxic, developers will start to look elsewhere. Like Microsoft, I'm sure Github can ride the majority-player wave for years before it starts really hurting them, but simply saying a community-centric company only has to look out for their paying community's interests is short-sighted.
[0] https://github.com/google/git-appraise [1] https://github.com/gollum/gollum [2] https://github.com/duplys/git-issues
If Github appears to go down the same road, paying accounts will leave along with open source projects; they are managed by the same people, with the same concerns for quality and corporate attitude.
If people get together and write up an open letter for you to fix things, you really should take it as a warning sign as the same people could easily get frustrated and move elsewhere or someone else could come along and offer a better product. It would honestly take only a handful of high profile developers to cause a mass migration.