On str, nothing stops a wizard doing this from starting as a fighter while neglecting str, then branch out due to the high int. Multiclass score prerrequisites are meaninngless when they can be avoided by singleclassing first.
A) You are stuck playing a 10 STR fighter for two levels. You will probably die. So there is the entire "hey party, I'm playing a fighter that can't hit things in melee!" bit.
B) This assumes the MC requirements stay only one sided; meaning that you aren't required to have both classes' MC requirements. I have no special knowledge of such things, but it would not surprise me if that is changed in the final PHB.
And I think that this Nothing is allowed until DM decides to is the most dangerous thing that could happen. I still insist that a more extense standard for play is desirable, and DMs still need to be encouraged to say yes, giving DMs the sole monopoly over character options will discourage the sales of lots of phbs, why buy something if all you get to use is already free? Or another one thought I hold, every single retroclone and their mom already gives the big four, and they are very easy to get right, but only DND can give the full experience and variety of races and classes and they aren't as simple, if the DMs are the gate keepers of all of that and the content for DMs is aimed at core four, then there is a chance we won't see many barbarians, warlocks and sorcerers this edition, and this might lead to blanket bannings or DM egocentrism which is a very real concern, I've already had to deal with it in editions which 'entitled players', I can only guess it will only be worse in an edition which entitles DM to intrude into the players characters.
I think you are overreacting and sort of missing the point about 5E, which, frankly, for a ton of people hyperanalyzing every little bit of 5E, is mind-boggling. 5E is a very basic framework of a game, with the capability of "build your own edition".
The game is designed from the ground up assuming that each group will make changes to fit their specific playstyle. Everyone seems to be hung up on the "official" rules, when the design of the game explicitly states that each group is intended to make minor and/or major modifications to make the game fit their own group.
OK with Damage on a Miss? Here's some mechanics that use it. Not OK? Here are equivalent mechanics that don't use it. Fine with people being able to heal themselves? Here are some mechanics for it. Not fine? Here are some alternate mechanics. The hard work in the edition is getting said mechanics to balance with each other, which for the most part, I think they've managed to do a real good job of it.
As for "DM empowerment", well, all they've done is shifted the burden to the whole group on how to play. By making multiclass opt-in rather than opt-out, it means that the game is balanced around non multiclass characters, meaning that players aren't screwed for not going an optimization route with multiclassing, as well as allowing WotC to not have to try to come up with ways to deal with every potential corner case of multiclass power-gaming. Making gaming groups sit down and customize the game the way they like it before they start seems like a pretty good idea to me, overall. People having "power" over each other in a gaming group is an issue between the people, not the game.
