I say keep the restrictions. Multiclass characters should be the exception, not the rule, and that means having exceptional ability scores. It's more like old-school D&D - not everyone gets to be a fighter-mage, for example. WotC seems to want an old-school feel to D&D Next. I imagine that's why the default method for determining ability scores is the dice rolling method, with point buy or array being optional.
Although I like the restrictions, the multiclass characters of AD&D were actually more likely to be characters with lower stats (as they made up for them with greater versatility) - it was the real specialist classes (e.g. paladin, ranger, monk) that had the most onerous stat restrictions. Human characters with two classes, on the other hand, were slightly hard to qualify for (and probably weren't worth it).
I do agree with those that note that a high-Str fighter isn't the only way the D&D Next fighter can be built...