That's the only difference and I (along with many other fans, as well as likely most GMs in baseball) think it may be insignificant. His lack of a proven MLB track record? Wouldn't the same criticism apply to Clay Buchholz or Phil Hughes, Ryan Howard 3 seasons ago? How about just his actual track record? Did you need an extensive track record to know that Jose Reyes belonged in the majors, or that Ryan Zimmerman could field at an above MLB level? This is the same conservative approach that so many people used when talking about Matsuzaka, as if he would immediately become a completely different pitcher when he came to the USA, despite his actual track record of success at a high level.
The fact that teams are willing to trade away their players with established MLB track-records (Lowell, Beckett, Pierzinski, Slocumb) for guys without a track record (Hanley Ramirez/Anibal Sanchez, Joe Nathan/Francisco Liriano, Varitek/Lowe) should indicate that what you are saying isn't even an accurate representation of how the game actually works. There is something to be said for MLB experience, it certainly is a better predictor of how good a player will be. However, there is also a time in a player's life when he has no experience but will be given the chance. At some point a team needs to make the leap of faith and just give a kid a chance to prove himself.
The "Hype" you talk about is nothing more than me thinking Ellsbury will eventually reach his career averages, which would be very exciting for a team that has struggled to replace Damon as a good leadoff hitter. He's not going to be superman, but he should contribute to this team and, I think, help them win a World Series.
What is bunk is assuming that all players are going to struggle with that adjustment. It is like talking about a pitcher who gets a lot of strikeouts in the minors or Japan and assuming that their numbers are going to take a dramatic hit because of the improvement of the league.
Buchholz may be missing bats by a foot and a half in AA, a foot in AAA and 6 inches in MLB. But who cares, as long as he's missing bats in the MLB? How would you even measure that sort of thing? The same is true for Ellsbury. You can see how he plays, he sprays the ball around with ground balls or linedrives, beats out a lot of balls in the infield, and turns singles into doubles and doubles into triples on balls in the gaps. He may beat them out by 10 feet in AA, 5 feet in AAA and 1 foot in MLB. Who knows?
He's not the type to suddenly stop competing and being successful if he struggles early. Again, I don't think he's going to be Willie Mays but he can contribute. That's not hype, it is as plain as day.
The only enthusiasm I have had is that I have advocated that Ellsbury could possibly contribute to this team this season and he should not be prevented because he is young or because he doesn't have an MLB track record. I agree with Pat Casey that he could become a superstar. A good portion of that would be because of his production on the field, but he also has a certain charisma about him that has been infectious at every level. Add to that the fact that he's playing in Boston, and add to that how Boston has treated it's home grown players who turned out to be good (Nomar, Youkilis, Pedroia) and the words superstar and icon could easily apply.