Savage Wombat
Hero
I don't think your character mechanics needs to be backwards compatible. You should be able to play the same concept in any of the editions though. You just need to change how you're going about it, and of course levels don't mean the same thing in each edition.
Just because you were a level 8 halfling fighter with a shortbow in 3.5 doesn't mean you'll be the same thing in 4th. You're more likely going to be a ranger, or the Essential Hunter version of the ranger.
Maybe before you needed to be a multi wizard, fighter, rogue to get what you wanted... Now you can take a roguish theme, on your swordmage who multiclasses into wizard via feats to get some extra spellbook fun.
Mechanics are different you just have to be willing to read the system to build your concept. It's not, nor should it ever, just be cut and paste of the previous edition (otherwise you're not a new edition, just a very confusing additional source book).
And that's sort of my point. If WotC had released a "conversion guide" that explained things like you did, and made suggestions on how to make a conversion work, some of the people who rejected 4e might have tried it.
Instead they just told us that it wasn't possible, start a new game. This may have been the truth, but it wasn't good marketing strategy.