I'd say there are definitely better, more theologically sound Bible teachers out there than Tim Keller. I'm not saying he's not a christian, though.
I read the Reformation Charlotte article, and I would disagree with his stance on theistic evolution, but belief in a young earth isn't essential to salvation (although from a scientific and theological perspective it makes no sense to believe in theistic evolution.)
As for "contemplative prayer", I'd have to know exactly what he means by that. I meditate on scripture, but it doesn't mean I mindlessly repeat or sing Bible verses. It just means that when I read the Bible, I ask God to speak to me through His word, and He does. Sometimes I contemplate a verse or verses for hours on end, whether I'm at home, work or wherever. I make it a regular practice to pray daily in private (I just talk to Jesus and He talks to me.) I DO NOT empty my mind and chant mantras; I think that's dangerous.
As for mystics, Roman Catholic or otherwise, I read them sometimes (in fact right now I'm reading 'Revelations of Divine Love' by Julian of Norwich.) I've read others, as well, like Madam Guyon and Richard Rolle. While I disagree with certain doctrinal points, I can say they were exemplary christians. To be perfectly clear, though, Roman Catholicism is absolutely heretical. There were and are true christians unfortunate enough to be caught up in it.
Social justice? It seems to me when I hear these words my mind immediately conjures up images of radical left wing crazies who think all white people are racist and who want a communist utopia. What does Tim Keller mean by social justice? It is certain that christians have new hearts and new natures, and therefore bear good fruit. 1 John 3:17 says "But whoso hath this world's good, and seeth his brother have need, and shutteth up his bowels of compassion from him, how dwelleth the love of God in him?" I can't say I am a christian and ignore need all around me. But the distinction is instead of being saved by good works, they are but one indicator of the new nature christians possess. If Tim Keller is saying we are saved by helping the poor or some other good work, I of course disagree. I don't see that in the statement the article quoted.