I usually circulate my builds with Janna, but I've been in matches were my teammates have been royally pissed off at DPS Janna because they try to chase her or defend a turret, but by the time they could catch up with her, they are at 1/2 to 1/4 of their HP, and if they pursue her, they will die, so they have to retreat, and hope that a turret it nearby.
Playing Janna effectively overall is knowing what build will help your team the most. If you are lacking in DPS or ranged DPS, sometimes it's a good option. If you have only one or no tanks, or no mage, you probably have plenty of DPS on your team already, and would want to build another way. You also have to pay attention to the other team. If it's a team of 3-5 mages, building Janna as a mage might be a bad choice since they will all have high magic resistance anyways, and Janna's magic damage will not be able to compete as well with other mages. Recognizing and utilizing Janna's flexibility in builds is the best way to play her.
What makes Janna effective in DPS, however, is her spells and her movement speed. (This is also why she's an effective support tank/cooldown tank. Give her Armor/Magic Resist/and Health buffs, and she becomes an almost indestructible disabler.)
When you use Howling Gale for its knock-up, she still attacks the enemy while they are in the air. (This gives her anywhere from 2-6 free attacks depending on her attack speed.)
Casting Zephyr does a bit of burst damage (more on non-mages, or enemies not carrying magic-resistance buffs) as well as slows them down, so you can start chasing the enemy, and since Janna is already fast, the enemy either has to burn a flash (which only sometimes works) ghost, a cleanse, or an exhaust or stun in the hopes that Janna can't catch up with them, but I've been in situations where this has happened, and I still managed to get the kill. A quick fix to that issue is Mercury's Treads. I seriously think that these are almost like the God boots of this game.
Sure, her overall DPS is lower than other specifically DPS champions, but I've killed tanks with DPS Janna who were at the same level as me and we both started the fight at full HP. (Janna is difficult to kill 1 on 1 for a large portion of the enemy champions unless they take her by surprise and use stuns/slows/disables so she doesn't have time to react.)
You already know her Eye of Storm buffs attack power on whomever it is cast, but even without AP, the shield absorbs 300 damage at level 5, regardless of what the damage is from. Also, half of the game, I'm using it to protect turrets, or to give tanks more survivability. Eye of Storm is extremely overpowered early game, so it can help get the enemies behind in leveling up/gold/experience, or help keep you (and lane partner) from falling behind.
Her Monsoon is also a decent healing spell without AP. (Compare it to the regular summoner heal spell. It actually heals more.) It won't necessarily save you from 3 enemies, but I've never had an ally complain about getting a free heal. Monsoon is effective at keeping creep waves alive at turrets, as well as helping out your teammates.
I apologize for this wall of text, but I believe in full support on ranged DPS Janna, as well as her several other builds. (I've done splash builds with Janna occasionally, and they also tend to be effective if used at the right time.)
I personally hope Riot does not buff her much, because if she gets buffed just a little too much, the following will happen.
1) Slews of people will start playing her, and she only costs 1350 IP.
2) Everyone will start screaming about how Janna is OverPowered.
3) Even more pointless arguments will start about how Janna is bad at blahblahblah and better at yaddayaddayadda.
Janna, in my opinion, is a pretty well balanced champion as she currently is. I used to think she needed a lot of work, but I've since changed my mind.
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