a) I agree with @aggroed that the value of a social network increases dramatically with its number of active users. Yes, you are talking about STEEM while I wrote "social network". I am well aware of the fact that the STEEM blockchain doesn't consist of Steemit (or a social network in general) only, but it is a fact that the masses (if they join) will connect to the STEEM blockchain through socially interacting with other users.

One big problem is that new users aren't supported well enough. Experienced members should do the job (yes, it's hard work) to manually watch out and vote for posts of new users instead of auto-voting friends only!
I have tried to convince many friends to join Steemit but unfortunately most of the time if I wouldn't vote for them they nearly wouldn't earn anything if if they are writing interesting articles, many comments or upload dance videos on DTube or DLive.

As many others already mentioned, it really has to be as simple and fast as possible to create an account!

b) We have to make self-voting, circle-voting and spamming less attractive, for example by ...

• ... thinking about a reward curve which started as n^2 / exponential (thus flat), and then later changed into linear which would work against self-voting as well as excessive rewards.
  @clayop had a similar idea.

• ... implementing diminishing returns when upvoting the same accounts (including own ones) again and again.

• ... reintroducing the restriction to four (or less) full paid posts (including comments) per day (from some hard forks ago) which was very reasonable. Just think at @haejin for example ...!

• ... considering also other ideas like the one of UserAuthority from @scipio.