I DM'd my first game a little over a year ago on Roll20 and have been loving it. I run an irl campaign now as well and actually prefer to have the group play in Roll20 with laptops, even as we interact face to face. It is an amazing feelin and keeps everyone more engaged and focused than any other format I've played with before.
That being said... there are some major roadblocks to DMing here:
1) Maps - There are lots of resources online to find some great free maps. And, of course, the Marketplace has some good resources as well. The thing is... I already pony up over $100 a year for the Subscription so I can use things like Dynamic Lighting, mobile app access and not having my players sit through adds when i switch between screens. So I'm very unlikely to spend more money. To this end, I actually learned a lot about how to use GIMP (a free open source program similar to Photoshop, for those who don't know) just to make the maps I wanted. Because as nice as things are online, it's not likely I'm going to be able to easily find a map that would be appropriate for exactly what I want to happen in the session. While this is a skill I'm glad I've cultivated, it's also one that has taken hundreds and hundreds of hours to do and it STILL takes roughly 5-10 hours to set up the maps (including the actual map itself, dynamic lighting, prop + object placement) every session. I don't have any suggestions on how to trim this back, but anyone who wants to use a map (one of the biggest strengths of Roll20, in my opinion) has a long uphill battle of experience they have to slog through to create the types of scenes they want.
2) Object Search - Okay, this ties into Maps, but deserves its own mention... the way Roll20 allows me to search for objects such as a table or a drgaon's egg to put down in my map makes creating sessions great. For either map building before a session or for on the fly needs, such as my player wanting to create a Spiritual Weapon in the form of a frying pan, the system is a great tool. HOWEVER, it has major flaws. First being that you literally cannot search for more than one word. I can type "dragon" and I can type "egg" but the second I type "dragon egg," no results show. Also, the quality of the results can sometimes widely vary, which I know isn't something Roll20 can directly control since it is powered by Google, just a roadblock of another thing a new DM could spend a lot of time struggling with.
3) Tokens - The logic behind tokens is great and allows for a TON of versatility. It's one of he other strengths of Roll20. But learning how to hook up tokens to character sheets seem a little daunting to a new player, as well as the little nuances that make it very important. For instance, just typing in HP on a token is a bad way to go, because if you move to another map and already have the token set up there, it will show full health unless you link it to the HP of the character sheet. There's no good way to cut down on this type of dependency, but some head's up of good token maintenance, especially between maps, would go a long way. Also... the Token little badges or status icons of whatever. GARBAGE. I'm really sorry to sound so negative, but every time I want to track a status like Frightened or Stunned or Prone, I just look at those nearly forty options and shake my head. What's a good Prone sign... the guy hunched over with lightning bolts coming out of his back? These icons are all way too vague and unhelpful. A complete overhaul with easily decipherable icons for common conditions across some of the more popular editions would be insanely helpful.
4) Layers - I'll lump Layers and Drawing Tools together here in one item, for the sake of space. Layers are great - it allows me to make the maps feel organic and alive by having items hidden from view or locked from being able to be moved when I want to select something. I really wish there was a way to access the Dynamic Layer through the Right Click function, like the other layers, but the Quick Key shortcut of "L;" works well enough for those who can discover it. My big problem with Layers is how Drawing Objects so completely and totally ruins this. Want to create a cone of effect? Well... I hope you like struggling to click something for a couple minutes and totally chop up the pace of the game! Put three medusas in a room together and try to figure out a way to show the cone of their vision in a way that the players can see and avoid and without making it impossible to move either the cones, the players or the medusas... drawing (or even using objects from the search) that need to cover a large area visually but still need to be moved make the map nearly impossible to manage. If there was perhaps a better way to have tokens generate cones or fields this would be easier, but as is, it is incredibly daunting for someone new to the DM system to do something that is remarkably common.
5) LFG - This has been harped on this thread quite frequently, but it's worth noting... the DM needs to do a LOT. They need to create a world, they need to know the system, they need to build maps, they need to know the rules... and, yet, where I can envision a new DM being put off and sent running for the hills is the LFG. Because in addition to be master architect, they also need to deal with the D&D equivalent of a Craig's List ad and the flood of strangers, each one with varying degrees of ability to read + follow instructions, varying levels of skill (either with the game system in question, Roll20 in general or a combination of the two) and, to be blunt, varying ability for basic social etiquette and niceties. Some templates on how to post a LFG (such as what questions to ask of potential players, the best ways to gather potential character information, prompts to talk about your style of DMing to players, etc.) might help on this front. Also... and I hate to suggest this, but... a rating system, for both players and DMs, to give feedback on inappropriate behavior, flaking out behavior of not showing up, overall experience, etc. would provide some piece of mind. I don't mind taking a Roll20 noon under my wing for a campaign, I just don't want to spend time getting to know a player, doing possible chats to make sure they are a fit, discuss potential character backstory and world building for me to find out that this person regularly applies to 20 games a week and flakes out on most of them. I always tell DMs new to Roll20 to over recruit, as the chances of you making it to your third session with all the players who said they would be interested is not high.
I really love Roll20 and think it does amazing things. For a new DM, though, combing through the Wiki, trying to figure out how to use the interface and becoming familiar with what your system's character sheet can do can be overwhelming. And while there are lots of resources out there in terms of Forum posts, Wiki articles and YouTube videos, there isn't a clear path for anyone to take it they have the thought "hey, I may want to be a DM."
If there was a catalogue of step by step guides, from the first steps on how to create your LFG to the most advanced concepts of how to integrate APIs, I think it would help tremendously. Each DM wouldn't need to follow the map to a tee, but having a map in general could help in knowing if you are skipping steps or are making mistakes without having to suffer the pain of going too far down a given path before things are FUBAR.
/end ramble post
EDIT: Also, just an idea but... what about some free One Shot modules for some of the most popular systems, with everything already pre-built, for those that subscribe to the Pro service? Something that takes the initial shock of "I have to build everything to even see if I'd like being a DM" away and makes the first experience a little easier? It could also work as a way to showcase the talents of some of the Marketplace artists/creators, for anyone interested in their future products. Just a thought.