Leaking. Tanks are still designed the way they more or less always have been and are susceptible to the same issues. RDA's still just have a hole on the side and, obeying the laws of physics, will pour out the juice when tilted. I don't care if YOUR tank has never leaked, or if I am "holding it wrong", it is still a potential and very realistic issue which holds back a lot of smokers from converting and is paid little attention to by the industry.

Some examples of innovations in this regards I would like to see more of:

* RDA airflow designs like the Vega RDA (SKU 9726141).
* RDA airflow designs like the Convergent RDA (SKU 9661717). On this one the air channels are raised slightly above the deck, so when you tilt the RDA the juice will flow underneath the air holes. The intention of this design might not be to prevent leaking, but in practice it does to some degree. Taking a bit further it could make for a pretty leak safe RDA.
* Use some kind of gel or paste instead of juice. This could kill leaking totally. I think this has been tried in the past with little success, but it could possibly work with more research and development put into it.