Oh this makes me sad. There are very few modern American voices on politics that usually make as much sense and are as well thought out as Andrew Sullivan's. It will be a loss not to hear his opinion everyday. His reasons for leaving are, of course, understandable.
I feel like Ta-Nehisi Coates Matthew Yglesias, and Johnathan Chait are the only other voices that come close.
Matt Taibbi is fun to read, but also shrill, predictable, and at times pretty clearly underinformed. I really like his political writing, though I don't always agree with his conclusions, but his writing on finance has often been just one or two notches more credible than ZeroHedge.
I generally always feel like Taibbi is deliberately writing to get a rise out of people. I didn't particularly like Sullivan --- my opinions of him hardened during the first term of Bush II --- but I did feel like he was trying to be careful.
I dispute the idea that it's hard to find similarly careful journalists today. What's perhaps hard is to find blogospheric gravity wells like Sullivan. This thread compared him to Ta-Nahisi Coates and Matt Yglesias, but neither perfectly fit Sullivan's mold. Coates doesn't cover politics, and Yglesias is an econ reporter. You have to assemble a Sullivan out of parts now, but I think those parts are of higher quality than Sullivan's (say) 2005 competition.
I feel like Ta-Nehisi Coates Matthew Yglesias, and Johnathan Chait are the only other voices that come close.