I could write a book about this question!
Firstly I think that there is a large learning curve - not only in terms of how to use the platform, but also in terms of ethics (such as using bid bots). New users have to get their head around the 3 different currencies (Steem, SBD and SP), voting power/manna, resource credits just to name a few things. Then there are the ethical things such as bot usage - how much is too much? What is considered spam?
Next off there is the fact that Steemit is very slow going in the beginning (remember unless your a celebrity then no one knows who you are when you join) - I think people get frustrated when they join and there posts make 0 or very small ammounts, and they see how much people make on trending (not always knowing the votes were paid for, and thus highly inflated!). Steemit takes time and effort to grow your account by getting to know people (this is something that new users dont always understand). There are loads of accounts out there that are on the search for under rated content such as @curie, @asapers and many more - this is something again not all new users are aware of.
And now with the new HF20 new users are facing more challneges due to the lack of resource credits and not being able to transact enough to socialise and get to know people.
Lastly I think a massive problem (even for me until recently) is realising that Steemit.com is just one site on the Steem blockchain, and thus there are loads of ways to make $ that arent all blogging! Not everyone is good at blogging but there are so may Dapps coming out for all sorts of people such as @dtube (videos), @dmania (memes/gifs), @actifit (fitness tracking), @steemmonsters (tradeable cards), and many, many more!
First and foremost if you're a new user or a user with very low steem power, your engagement level will be limited this is because the new HF20 has brought changes to steemit.com and with the introduction of the RC (resource credit system) it may be difficult for you to make transactions on steemit transactions like commenting, resteeming posting and wallet activities as well.
This brings me to the fact the second challenge you may face is not getting support without the maximum kind of engagement, whether you like it or not steemit is decentralised and this means the people in the platform determines whether you earn or not, influential people tend to flock around influential people which means that as a upcoming or growing users you will need to be more than extraordinary with your level of involvement with people and you will definitely have challenge with choosing who to follow because so many people are now on the platform and before you can find the right people it may definitely take a whole lot of time to choose people who really want to engage with you and help you grow.
Another challenge is content creation, people always have a challenge of creating contents that will attract support, expecially if you do not have a niche or the niche you're trying to create is not really plausible or attractive, although content creating is not necessarily the only way to earn on steemit, but it's a way to introduce yourself to steemit community and content creating may be a challenge because ordinary contents don't thrive unless you've invested so much in steempower.