One thing that is very annoying with a lot of game systems is that whenever a new supplement comes out, the new classes or abilities in the supplement are better than the ones that came before, creating an endless arms race. Now that I have the forgotten realms players guide, I might as well try something risky and see if I can guess how good the sword mage class is before having a chance to play it. I'll perform a comparison with the other 2 defender classes. To keep my sanity, I'm limiting my analysis to level 6, comparing an Int/Str shielding sword mage with a Cha/Wis sword & shield paladin and a Str/Dex flail & shield fighter.
Weapon: All 3 classes are happy to use a bastard sword. The fighter is clearly best with his +1 bonus to hit. The sword mage may get a permanent +1 to damage if he is allowed to use his bastard sword 2-handed while attacking and one-handed while defending; I'm not sure how this is supposed to be interpreted. Even if he isn't allowed to do this all the time, he can still do it whenever he has no use for the extra armor class. The paladin is clearly last as he is nothing special in this department. Also, the fighter is best in terms of basic attacks like charging, and the paladin is the worst.
Defenses: The paladin beats the fighter with his free plate armor proficiency, extra point of special defense, and his extra healing surge. The sword mage is more different. He will probably have an armor class equal to or one less than the fighter with a shield. Compared to the fighter, he also has the disadvantages of not getting the shield reflex bonus, of having his natural bonus be in Will (the defense least often hit), and having one fewer healing surge. On the other hand, the sword mage has one higher movement, has no skill check penalty, can more easily upgrade to better armor, and has a free hand to do all sorts of useful things with (like using consumables). So maybe the sword mage is equal to the fighter and the paladin is best.
Skills: The fighter has one less skill than the others.
At-Will Powers: The best powers of each class seem roughly comparable, none seems overwhelmingly better than any other. But since you get 2 powers, the sword mage is clearly best since he has a richer and more versatile selection of powers.
Encounter Powers: The level 1 encounter powers of the fighter in paladin are both fairly mediocre, the sword mage is a bit better on average but not exactly devastating. All 3 classes have super powers (powers far superior to ordinary powers) available at third level (Rain of Blows, either paladin smite, and Transposing Lunge), but the sword mage power seems less good because it is less convenient to use. If the 3 classes don't choose to take superpowers, the conventional powers seen more or less comparable.
Utility Powers: The fighter has some useful but not spectacular utility powers at level 2, and a great power at level 6. The paladin has some crummy powers at level 2 and a good but somewhat restricted power at level 6, but he also has a really useful utility power at level 1 in the form of divine channeling, so maybe he compares favorably to the fighter. The sword mage has some really cool powers at level 2 and good powers at level 6 also. All the powers are so different that it is hard to figure out who is best overall.
Daily Powers: All 3 classes have normal daily powers at level 1, not much to distinguish them. The conventional level 5 powers look somewhat similar, but neither the sword mage nor the paladin have anything to match the awesome superpower of the fighter (rain of steel). However, the paladin has the very useful lay on hands ability, so I would say he is second and the sword mage is third.
Defender power: Who is the best defender? This is tricky. The fighter has the hardest time marking the target (he has to be both adjacent to an attacking the target), though at least it doesn't take a minor action. The sword mage has the easiest time marking the target, he doesn't have to do either. The paladin's power, however, goes off on misses as well as hits so it will probably cause more damage than the shielding sword mage prevents. But then, the sword mage is actually doing a better job of protecting the target. The fighter can have his power disrupted more easily. He also has this advantage that his power is an improved opportunity attack, while the other characters can use their powers in addition to opportunity attacks. The fighter can occasionally mark multiple targets, which the others can never do. But the fighter is the only one who has the incredibly evil ability to lock down the target so that he cannot even shift. The question is, how easy is it for the sword mage to avoid engaging the marked target and thus get his power to trigger? I'd say the paladin is the weakest of the 3, and I'm not sure but I'll guess that the fighter is still on top.
So it looks like they did a good job, it does not seem like the sword mage is clearly superior, it seems comparable to the other 2 defender classes. It remains to be seen whether my analysis holds up under play.