The biggest thing about forum mafia, as has been mentioned, is time.
This becomes true for both scum and town alike, but you can craft your posts. In real life games it's all reactionary. You accuse, or you're accused, or you talk, or whatever. You're getting instant, unpredictable, unrehearsed reads off people. Here... well, because a response may come instantly or in 12 hours, you never know. Maybe it was written word for word to draw a reaction. Maybe it was spontaneous to when that person logged in. The beast is still the same. It's still Mafia. But how you succeed or fail is very, very different than in person.
As a prime example of the above example, back when I was still active, we were playing one of our first massive Role Madness Games - that is, everybody had a role. Day one was the relative scuffle - everybody calling everybody out, some cases being made, but no real evidence produced itself. I was town. About 42 hours I noticed a pattern on a certain player. He stayed mostly quiet until called out by name, and then within 6 hours or so he consistently posted. This drew me to make a case on him being scum trying to "slide by" unnoticed D1. I was vocal, time was dwindling, the lynch went through, and I was right - he was scum.
Was it a great case? No. Could there have been legitimate reasons for his lack of posting? Absolutely. But I had my read, it was as solid a case as anything D1, and it stuck.
His alignment reveal gave everybody still left in the game the following day a LOT of information to go on. Who defended him? Who tried subtly, and not so, to derail his lynch? Who jumped on early as a potential bus (scum lynching their own partner)?
In a real life game everything happens so quickly that there's often not time for that last minute irrational decision. I know it sounds backwards, but with so much more to analyze you've already decided if somebody is guilty or not in the first 5 minutes of real life play. And reactions to your thought process and belief drive your opinions of others, with or without the alignment/role reveal.
Here, because of the community we are, it's as much about the puzzle as it is the game. Everything gets analyzed, fairly or not. It's very much a different game, but it's still just as much fun, intense, frustrating and demanding as it's real life counter part. There's a learning curve, sure, but once you've conquered that, it opens up whole new doors - and makes the real life version look like child's play.
- Note, I'm a terrible poker player irl. I know my math, I know my cards, I know my hands and my odds. I can't keep a straight face to save my life. I cannot, for the life of me, play mafia in person. I'm either an innocent child (obv town), or scum - and everybody knows it. Thus forum mafia is perfect for me. Give me time, and I can craft any post. Make me say it on the spot, and, well...
Forum Mafia is different. It's still Mafia, no doubt, but it will take some adjusting to. Fun it certainly is... But don't expect things to be as cut and dry as they are irl.