On swordmage:
You can afford to be underwhelming with Strength because you can take the feat that lets you use your Int for basic attacks. Combined with the powers that can be used in place of a basic attack for charging and Aegis of Assault and you'll only be using Strength for Feat Prereqs and bonuses to certain powers.
Con affects your other power choices. For feats, it can allow to get Hide Prof and Hide Spec at paragon tier. One posibilty is using the new weapon from Adventurer's Vault that is BOTH a heavy blade and an axe, in which case you can use the CON for axe feats.
Dex doesn't affect your power choices, and won't improve your defenses. The primary influence will be feat selection. One thing to note, in addition to Arcane Reach, the heavy blade feats require Dex in addition to Strength.
As for Will Defense, you get a bonus from the class, and ultimately, will is the least often targetted defence by the creatures in the Monster Manual, so it can afford to be low. Unless you want to go with the Wandering Swordmage paragon path [which requires Wisdom], you won't really make a lot of use out of the stats.
There is also skills to take into account. You may want good [or at least ok] wisdom for skill purposes. It's ok to have one person in the party that is charismatic, but multiple characters with relatively good perception/insight/heal is not a bad thing by any stretch of the imagination.
Also, don't forget to choose your (first) elemental manifestation. Most will give you a bonus to one of your defences, some energy resitance, etc ... Each one has an encounter power and some associated feats.
For your feat, there are a few others to consider [although maybe at later levels]. If you significantly reduce your STR, you may want Intelligent Blademaster. There is Elemental Assault which adds 3 of your manifested element as damage to the basic attacks you make with your interupt basic attack. There is also Escalating Assault which gives you a +1 to hit with the Aegis of Assault basic attack each time you hit with that power. [To a total of +3]. This bonus lasts all encounter. So, depending on whether the Aegis gets triggered a lot or not, those powers can be VERY good. Having another defender is especially good as he can possibly stand between you and your marked foe, enabling you to trigger your Aegis more often.
If you are going that route, upping your AC can help make your allies better targets than you, even with the Aegis and marking. So Hide Armor Prof [and enough Con to get the Hide Spec at higher levels] could be the way to go.
Opportunity attacks are really a matter of DM play style. Certain monsters won't risk it, others will. There are a few ways to "force" them, but those are rare powers. If the DM is the type that would not have his creatures provoke OAs, you can save those feats for later, if at all. As a Genasi AND a swordmage, you still have plenty of feats to consider. Ignoring the OA feats means that you don't really have to worry about heavy blade feats until you reach epic tier, so Dex isn't as important [unless you want to get the feat at that level to increase the critical range].