What advantage do Mt.Gox or Kraken have over Coinbase? I just verified at Mt. Gox and am pending verification at Kraken.  However, I quickly realized that there were hurdles that I needed to jump through in order to get my USD into BTC.  Coinbase was a much easier alternative. 

What advantage do Mt. Gox or Kraken have over Coinbase?  I realize it's quite easy to move BTC into both Mt. Gox and Kraken hosted wallets.

Is it purely anonymity, pricing, or something that I'm missing, like trade execution features?
An important advantage they have over Coinbase is that they can offer a service to other people than U.S. inhabitants.

mtgoxUSD may be by far MtGox's largest market, but don't forget that mtgoxEUR and mtgoxJPY are still the 5th and 6th largest markets that exist. More trade volume means more robust and stable prices which is a good advantage as trader.

I'm not familiar with Kraken, so I can't speak for that.

Also, Coinbase does not support limit orders. This is very important. Read the link or search a little about limit order and you will understand.