Invisibility allows you to use non-attack powers, take a variety of actions such as disarming traps or activating magic items, gives you total concealment, and immunity to opportunity attacks. It is indeed very powerful. Remember, wizards are not the primary focus of the game anymore.
Something to keep in mind: A lot of the old favorites were old favorites because they were absurdly powerful, like Time Stop.
While I agree that Time Stop was ridiculous, Invisibility is not "very powerful" if the user cannot attack and keep it going. It's not even close to powerful. On the optimization sites, it's rated a 2 out of 6 (as is Dimension Door) and rightfully so (course, most of that is due to the fact that it's a standard action to use and to sustain).
Yes, a PC could sit there in his little invisible bubble and do the things you say, but so what? If a foe had a 50% chance to hit him, it dropped to 25% for 2 out of the 4 types of different attacks (ranged and melee, it doesn't help against close or area). So, the equivalent of maybe +3 or +4 (encounter dependent) to defenses. He's still not really that safe. Plus, invisibility does not prevent everyone from knowing exactly which square a PC is in. That requires a successful Stealth roll which Int Wizards rarely have an abundance of.
And is immunity to OAs that impressive? Not really. OAs very rarely happen in the game as is. Once players figured out how to avoid them manually two plus years ago, they have become an infrequent occurrence. I cannot remember the last time someone in one of our games provoked an OA unless it was done on purpose for some reason.
Teleport gives one immunity to OAs, but it's handed out at first level to most of the classes. Why isn't teleport considered potent if it has immunity to OAs?
And the Invisibility power is a Daily power. A DAILY. Let me repeat that. A DAILY. It should have some umph to it, but it is a sad sad power that very few players ever take (just like Dispel Magic).
Invisibility is the D&D equivalent of taking your shoes off at the airport. A knew jerk overreaction of mega proportions that was never well thought out, just implemented.
Face it. Everyone has a different opinion of what is overpowered and what is underpowered. But one thing that we should be able to agree upon is that just because a power is written in the books doesn't mean that it is balanced. There are a plethora of optimization guides on the WotC site where there are a ton of powers that are illustrated as seriously underpowered or overpowered for their level and many of the overpowered ones have been erratta-ed by WotC.
What this illustrates is that the 4E game designers, for all of their good intentions, are just people like the rest of us. People who make serious mistakes. I think in their effort to water down Wizard, Clerics, and Druids (i.e. CoDzilla), they made a lot of serious overreactions when nerfing spells (similar to taking your shoes off at the airport) to the point that for the most part, many canonical spells for these classes are now a bit of a joke.
Teleport is handed out like candy, but Flying and Invisibility are fairly non-existent (at least flying was until the Sorcerer came out, then Sorcerer only one round flying made a bit of a comeback).
Dispel Magic is a perfect example. It doesn't do anything now. Most monsters don't have zones, they have auras instead. And how often do monsters throw out conjurations? WotC's answer to how worthless it is was to change it from a Daily power to an Encounter power. Granted, that at least allowed the Wizard to almost always have it available, but it doesn't change the fact that zones and conjurations just don't show up in the game that often.
300 creatures out of 3800 of level 6 to 30+ have either a zone or a conjuration (350 out of 4800 total, but 6th level PCs rarely run into lower level monsters). As written, it's a Standard action spell (i.e. it uses up precious action economy) that is useful against less than 8% of all foes, and there's a 40% chance (or more against the really tough foes when you really need it) that it doesn't help. That means that it helps less than 5% of encounters or about once per two gaming levels.
So from level 6 to 30 (2.5 years of gaming for some groups), the spell helps maybe 12 times or once every 2.5 months. Very few people are going to take a spell that helps out that infrequently and takes a standard action, but doesn't do damage, if they actually know how infrequent it helps. Usually, they find it out via trial and error by having the spell for 9 levels and realizing in the last year, it only helped them a handful of times and they replace it.