This has probably been discussed many times before, but I am trying to ascertain whether 4E classes have distinct "tiers" in terms of how powerful they are.
Not really. If they do, then they are very shallow tiers, without a very big gap between the most powerful classes (the Ranger might be a candidate) and the least (perhaps the Seeker, ironically, the game's other archer archetype, or the Warlock). Nothing like the gulf that existed between casters and non-casters in prior eds, anyway.
The swordmage and the rogue have been far and away the most powerful of the group; I am having a hard time differentiating what is tactical skill on the player's part and what is a possible imbalance in terms of class power level.
I don't have a good sense of the power of the swordmage, not being into the 'Realms, but the Rogue is a very strong class on a few points - most notably, it has a +1 unnamed attack bonus with a +3 proficiency weapon and a wide selection of weapon attacks that target non-AC defenses, meaning the rogue hits /very/ consistently, which is a telling advantage in 4e.
As a general comment, one thing I've enjoyed about 4E is that it seems the onus of character power is less on system mastery - how well you can tweak the rules to maximize your character - and more on tactical mastery - how well you can utilize the powers your character has. Is this the experience of others as well?
I hadn't thought of it that way before. I mean, I noticed that 4e was even a little more tactically oriented than 3e, and a lot more robustly balanced, meaning you didn't 'win the game at chargen' so much as you did in 3.5, but I never really put the two together. But, yes, that's very true, how effective your character is in 4e tends to have a lot more to do with how you play it than how you build it. Not that you couldn't intentionally build a bad character, but it's pretty easy to build one that's good enough to do very well, if well-played.
The only thing I'd add is that /group/ tactics are also a big part of it. If the defender and striker in your game seem really powerful, it might also have something to do with how much the leader is buffing them or how well the controller is setting up their enemies.