I don't doubt one moment that MS has changed the system that was leaked hardware wise. Maybe not extensively, but they probably did at least a bit. The thing that concerns me is not necessarily the hardware in which I have faith in they'll make a decent performing machine at the very least, but it's initial reliability. The 360 was the least reliable gaming machine ever made. At least the "flipped upside-down" PS1s and "laser tweak via screwdriver" PS2s usually occurred after some time, and usually after extensive use. The 360 on the other hand was RRoDing day-one in oddly high numbers reported from forums, then after a week, a month. RRoD was scary to those buying the machine 3-4 years later that it would happen to them shortly as well. MS covered it up at first, via the "trust" people had in Peter Moore, and the media was too focused on trying to run Sony out of the industry, and thus articles about it were very "passive". I would like to think with the basic hardware that sounds like is going into the 360 we shouldn't have these issues, but you just don't know about their QA and thermal designs.
Something else that's of concern is their software support for gamers. MS was guns blazing for the gamer in the early part of the 360 life but then dissipated mid-life and out. They don't have the studios in number or quality that say ninty and especially Sony has to rely on a steady stream of titles until the end of run for the console. The "money hats" started to thin out soon as they thought they had a large enough lead to not worry. Couple in that there maybe more ads than ever, along with the distinct possibility that they may not have the "3rd party port go-to" box this time it's easy to be very-very skeptical.
Of course this thing will be a media box. I see people bring that up as worrisome, but integrating 3rd party non-gaming related media services is a MS expertise. And it's not like the PS3 didn't have non-gaming media features, along with that brief shot at the past meeting shows that the PS4 will carry over and add more of these features in at the same time.