Digesting the playtest further and reading a long stream of posts on these forums, I'm finding Spell Combat to be lacking a bit.
It seems as if the design is trying to tackle the "trade off" factor of being a fighter/magic-user and really keeping that trade off as the central theme of the character. This just seems unsatisfying when you are trying to blend mechanics with a theme.
The idea of a TWF-esque routine where you cast and attack in the same round seems really cool, but it also raises all sorts of issues with the action economy that the system previously only had to deal with at higher levels. Now those wrinkles are emerging at low levels and the stacks of penalties involved don't make for an exciting low level ability.
Rather than wade into that issue, I just thought perhaps the whole thing could be sidestepped by shifting the rule exception to concentration checks.
2nd level Spell Combat special ability could be reworked so that instead of it doing this semi-TWF, instead it just allows auto successes with casting defensively. Basically, the thing that makes the Magus stand out is that his unique training in both combat and magic allows him to avoid attacks of opportunity, just as Improved combat maneuver feats allow you to grapple, trip, etc without incurring AoO.
The Magus would then be special because it would be the one class that could wade into battle, do his stabracadabra stuff, without risking his precious spells or getting critted in the process. This approach would capture in the mechanics the reason why it's cool to play a Magus, rather than a constant message of "you both suck at combat and magic!" being the main theme of the class.
Another real boon to this mechanic is that it would remove a lot of concentration check rolls, which means less die rolling, which means faster combat. The current Spell Combat ability means we'll be doubling our rolls and making combat take that much longer.
