Interesting questions, and good ones too. 
I don't really enjoy fantasy RP's much, or ones that heavily involve magic, so some of it isn't as relevant to me. But I still have input based on being a roleplayer in general that might be useful and relevant. 

Regarding writing; I tend to vary the length of my posts depending on what the situation is. Does the other person I'm interacting with give me a lot to work with? Is there a lot to bounce off of there with interactions, questions, descriptive language (e.g. expressions, body language, etc etc). It all depends on what the situation is. Sometimes interactions are short and quick, sometimes posts are the same. There's sometimes only so many words you can wring out of something. Sometimes a door is just a door, sometimes it's a forboding portal of blackened oak, bound by iron as dark as the night. But I'd hope anyone I RP with is making an effort to be descriptive, so I'm getting immersed in the moment, and it feels exciting and enjoyable to read their posts as much as my own. 

Art types don't often bother me. I can't say I've come across an RP where the particular choice of images for refs has put me off - at least, for characters. 
I've been put off of mecha RP's because I don't like the aesthetics of the mecha, but that's also to do with how they're utilised and fit into the setting, not just their looks. 
I will admit if someone asked me to use a photo reference for a character I'd find it difficult and a bit off-putting. But otherwise, not a huge issue for me. 

As for the careers - I can't speak about a world of magic so much. As I said, I don't really do fantasy or high-fantasy RPs or things that heavily involve magic at all. 
But I suppose from an objective point of view, and not knowing anything about how the magic in this speculative setting works - it would depend on the mechanics of the magic, how it works, and how it develops. Are there different schools or types of magic? Are they related to talents inate to a character, or are they learned and honed by skill and training? Because if it's more the second, then careers and the experiences from them may have more of an influence on the characters' magical proficiency and ability. If it's more the first, then I suppose it depends on if their career choices were important to their personal development and the history that's got them to the starting point of the RP, and if those skills are going to be relevant throughout the RP or not.
As a GM, it can be useful to know how a player character got to where they are, as there might be strings in their background you can pull on to give them sub-plots, and to know that if you're setting up a problem for the player party, there's someone who can solve it using their skills, from whatever professions they may have had. 
As a player, I think it's good to have at least the broad strokes of their history involved. 
Even in a world of magic, if there are 'mundane' jobs, like you describe, presumably that's because those things still exist and are therefore at least useful and part of life in some way that could potentially be useful to the players? 

Regarding education of characters - personally, I can think of nothing worse than playing a character who is in a scholastic environment, so I'm not going to be much help there. 
As for learning things, everyone learns a lot of things both through formal training and education, and also by just trying things. For example, IRL; I have a degree in English and Creative Writing, and I've worked in admin for various companies in different sectors. However, in my own time, I have also done DIY and house maintenance, visited many museums, read many books, learned to cook, learned how to repair my bicycle, and how to shoot and maintain airsoft guns, and how to move in a group tactically (to a degree, not at all professionally, but it's still [i]knowledge[/i] that I can impart to others). 
All of those things are skills and information that I've picked up from a variety of places, and through lived experience. 
I suppose it depends how much you want to 'formalise' skills, as opposed to just narrative, free-form skills as part of a background.

Same with the 'battling' system. I never formalise combat or character creation with points and dice rolls in forum RPG's; everything is freeform for me, as I find the narrative flows better and I'm not a big fan of 'crunchy' rules, I prefer 'fluffy', narrative based stuff for forum-based RP.