Part of the problem with a full on reboot is shelf space, DC doesn't want to give up shelf space because Marvel (specifically), Darkhorse, IDW, and then independents will put out something to cover those spots. Once they willingly give it up they may never get it back. The second problem is doing a full on reboot with 50-60 titles starting fresh is also a problem, team books would fit in awkwardly because they would by and large be composed to characters just being introduced in their own titles. You could get by with say an aging JSA, and Green Lantern Corps (of aliens), but most of the more recognizable teams wouldn't be around just yet. The exception of course would be start in the 'middle' of a character and not at the very first day (which really doing that with every character would be odd anyways... then one day all these super heroes showed up).
Marvel I think had a decent idea with the Ultimate universe, I do think they intended it in some way to take over more then it has.
What I think they should have done was started wrapping up and stagger it out. Have say about 15 titles wrapped up when they started the new DC, and have 15 titles take over at issue 1 starting the new DC. Then have other titles over the course of a year or two wrap up and then be replaced by the new DC. Make sure both are clearly marked, I wouldn't put out any limited series for old DC to make sure people aren't confused.
I realize I may have said some of that before, not sure if I had hear or not. But part of the reason I bring that up, it would have made inserting Cyborg into the Justice League a little smoother instead of this some stuff is there but not all which just causes more confusion. Also fans of the older stuff can at least say, it's the new DC. Whereas now it's the sorta new DC.