Maybe I can make my unhappiness clear by looking at what 4e MCing doesn't give you. It doesn't give you at-will Powers, or (with minor exceptions) other non-Power class abilities. What does this mean?
If you want to MC into a defender you:
1) don't gain Armor, HP or Healing surges: you don't get any tougher, a key defender characteristic. Sure, you can spend even more feats on those things, but then you would have been better off not MCing and getting those feats instead.
2) don't gain the ability to defend allies reliably. While you do get the ability to (possible miss change w/fighter) mark 1 person/encounter with the initial MC feat, and might be able to pick up other defender abilities as encounter powers, your defender abilities will still be negligible. MCing with fighter, you *don't* get the fighters AoO root/anti shift abilities. WotC claims to have made paladin marks relatively unstable to prevent coward-pally tactics. With many enemy, long duration, mobile encounters supposed to be the norm, a single, weak per encounter mark+the option to spend a feat on a per encounter ability just isn't going to cut it.
It is lack of toughness improvements that really galls me: MCing into a defender, by costing feats, effectively *weakens* your defenses (those feats could have been spent on increasing your own defense). The inability to pick up significant party-protection abilities doesn't help any either.
If you MC into a striker: (the following is written under the assumption, borne out in the very limited info we have, that a striker's extra damage comes from their abilities that give extra damage dice, Sneak Attack/Hunter's Quarry/Warlock's Curse)
1) You don't get significant extra damage: you get 1 use of the striker's extra damage ability/encounter (unless you are a warlock, then you lose completely). Given the goal of long fights with many enemies, you aren't gaining even weak striker damage.
2) You aren't getting much mobility, the other striker feature. The warlock can get a per encounter movement ability with his MC feat. All can get per encounter movement abilities with further feat investiture, but 2-3 movement abilities/encounter, with 4e encounter durations, won't play like a high mobility character.
MCing into strikers won't get you either the flavor of a high DPS character, or the flavor of a high mobility character. Yay? Also, like the defender MC armor issue, MCing into strikers doesn't seem to net you weapon proficiencies... which can pose problems. Forcing extra feat expenditure to be able to use the weapon your MCed class requires is asking too much.
The cases of leaders and controllers is less clear, because we don't have much information about higher level healing, or any real idea on how the controller is supposed to control the battlefield.
