Janna isn't a pubstomper in her support role. At the end of the day you can play her flawlessly and your team won't make it work.
Most of the work is already done for you- because Janna is an ability based support (as opposed to an aura support like Sona or a tank support like taric and alistar) your two most important abilities are your ability to cast spells often (early game it's mostly just mana management but late game you will want CDR. Your best bets are the philo stone building into Shurelia's and then maybe a mana manipulator into a soul shroud, but I wouldn't over look items like Randuin's and Frozen Heart. Different team comps and different characters demand different item builds) and then your ability to put some oomph into it.
No, it doesn't mean gear like an AP caster, but it does mean picking up items like Rod of Ages, Morello's Evil Tome, WOTA and Zhonya's Hourglass are not bad ideas. Past mid-game if your damage spells don't actually hurt and your damage shield has no health to it you're just one giant liability.
No, this doesn't necessarily mean that you should ignore aura items like Aegis and so on. It just means that on her own Janna already has a kit that does the utility thing. If you wanted to play an aura machine you should have picked Taric or Sona.