I've looked at a few of the Janna builds and can agree with what most play out with, mostly. Primarily what I see in Janna is:
Excellant support!

My build gives her what she (what I think she needs) needs: Good hp, great mana, decent ap, decent armour/mgr for survivability, Tenacity (35), some speed (if you are a lichbanefan). Building to assist/save, if your looking to godtank/godnukepwn look elseware.

I have seen some AD builds, but I think you will screw over Your team with it (higher level play, if your team sees Janna they think support, not carry dps..). Seen tanky Jannas, you live long time but do not much. Pure caster, do lots and die real fast. And support, live good, help team, saves some whales, disables/slows all around!

I am to lazy to spend epic time into pictures and colourful hints, if you want hints its pritty self explanitory, shoot the tornado to disable/gank, use the ult to disable/heal/gank, slow to slow, shield to shield/assist dps boost AD kills(towers uncluded). If you want more information about them go look at some of the other builds, they have good builds and very good info about Janna's gameplay (beyond my general speedy-outline above).

Items:
Merc treads - you are fast, instead of being real near needlessly-speedy add some survivability and tenacity! Yahy!

Catalyst - Stay in yar lane longer!

Tear - Use more mana and gain more mana! Yahy!

Manamunipulator - Probably not the right spelling but bit of mana for everyone! Yahy!!

Aegis of legeon - Survivability and love from your team YAHY!

Cata=Rod of the Ages = Hoorah for survivability/spammability and more healing/shielding power!

Tear=Archangel = Yippie no more mana issues and tons of mana! And More healing/shielding power!!

Four items! Maybe if you got Manamunipulator throw in a soulshroud and get more tanky and have some CDR and help the team with it too!! Woot, casters now wuv being around your ****ty assistness more.

Fifth item... I thought... I have hp, mgr, a bit of armour with passive 77.4/lvl18+masteries.... Got mana in heaps... Ap makes my team happy, resistances are nice, movement speed is nice, mana is nice, some damage ability would be nice.. Lichbane? Sure! Now your a support that does decent few shot damage with lots of things that help you out!! (my masteries/runes are my style, if you try this build, best to focus on teamplay/support/survivability with them, mine do and work well for me).. Some good items after could be, aphat, abyssl, guardianangel, Bv, reveire...hourglass...willoftheancients.

Core:
Merc Treads
Aegis o L
Rod of Ages
Archangel

Decently cheap!

Your mana, your survivability, your team helpingness ontop of heals/shields, no real need to ever go back. I like lichbane in this build, you might not, I can never get to it... so its not a big deal. If your being focused then your doing something right, get soulshroud if you need beefyness and want more team helping or CDR, get BV is you want survivability and mgr, apHat for more happy heals and shields, really what the situation suggests.

Btw the items after the core I mentioned are things I think could work good for ending items, primarily if it helps the core Janna roles I think are her primary bounces. I added wards, sight wards, and oracles because being not item dependant you can do this without much loss, sure its great when everyone chips in, expesially he who has been fed by you.. But be honest, if your doing decent, lay some wards, use clarvoiance and have map awareness, its nice. Also getting Oracles means their stealth hate and want you targetted, bringing the 'hard to kill Janna' into play, and bringing focus off your carries.

Feed your carries, save everyone, and die little = Janna