JBJ will never steal 20 bases in the big leagues. I'd be surprised if he was ever allowed to run 20 times a year. He stole 9 bases last year combined between AAA and MLB and was caught 7 times. This man can not beat AAA catchers on the basepaths, he should be tried only very conservatively against big league catchers.
JBJ is a smart defender with great athleticism at the corners and decent in the middle but he doesn't have elite speed, and the power potential remains to be seen as well. All we know is that he's got really good plate discipline. Which is a really good place to start. The rest is up in the air.
I've said this before but my comp with JBJ is David Dejesus. That's probably a disappointing name for some people who want to expect Mike Trout or something, but David Dejesus was really solid in his day, boasted a lot of doubles power if little home run power, and as long as he could hit over .270 was borderline elite as an on base hitter. That's a package we should take out of a rookie if we can get it.
I get that with Ellsbury gone all hopes swing ti JBJ but honestly I feel that isn't fair to him. He's still got a ton of leaning to do about how to be a big leaguer, and the tools package isn't close to Ellsbury's -- he's going to have to be a lot smarter and more selective to produce at the same level, that's just a fact So it won't do to overrate him and set ourselves up for disappointment, and if you predict a 5 tool guy out of JBJ, that's exactly what you're doing.