A few trains of thought running on this one. Not sure if this post will entirely express them sufficiently, but, for some bullet points. 

[list][*]Posting deadlines help to keep a person [me] honest and sorted about my posting activity. Though, given the whole adulthood thing, flexibility is also appreciated, especially when work gets busy for me. Most GMs have been fair and lenient, and I've yet, over the past 3-5 years, been singularly bounced from an RP due to a delayed post. Most of the time, if I'm not locked in, I'm not alone in that regard as others have also hit snags. 

EDIT: An ancedote regarding deadlines. On one of my old haunts, there is a single RP running in the forum's RP section. It started in 2018. Despite the pacing and frequency of posts coming to a literal crawl (at times, months pass between posts), the players and the GM remain invested in it. No stressing about getting posts in by a certain date. There seems to be an unspoken approach of letting life dictate the pace of the RP, and I think there's a lesson worth learning there. 
[*]I can go either way with Face Claims. Keeping in mind that they only amount to what my character looks like, and, do an extent, their style(s)/fashion sensibilities, makes it easier. Though I do feel that the necessity of identifying a face claim by name can be a bit of stumbling point. On a few occasions, the FC that I find is some rando I found on a Google search, but the FC... err... 'culture' tends to demand a name, which means having to think of specific celebrities or public figures, rather than being able to just go with the first person I find that looks the part. But it all depends a lot on what sort of genre I'm working in. Modern and/or SoL affairs are easier for Face Claims than, say, purely fantasy settings. Otherwise I kinda just try to read the room if it's not specified: if most/everyone is using RL faces, then I'll do the same. 
[*]Lately, I've found that 3 characters is a good number for me, if I'm finding myself needing/wanting a good self-sufficient story arc; optimally, running a main character with two slightly-less-main characters. Having those relationship dynamics completely under my control alleviates the need to rely on others being on board with my development goals. Which I reckong should help in terms of developing relations with other peoples' characters without the need for them to completely come to me on things.
[*]Superhero RPs seem to be springing up a bit lately. And I'm very much into riding that wave. But otherwise, personally, I wouldn't say no to Urban Fantasy, with or without other genres mixed in.
[*]EDIT: My apologies for making this more of a pet peeve air-out. Speaking as a player, my best RP experiences have come under GMs that engaged (collaboratively and socially) with all of their players. If there's to be one person that [b]anyone[/b] can go to to talk story arcs, or hit up for a collab, it [b]should[/b] be the GM. Having been in a handful of situations where this did not prove to be the case (and having been on the receiving end what felt like exclusive and directed apathy), that's the kind of experience that just really sours this little hobby of mine. And I get it, some people are just a bit more hesitatant to work/socialize with those that they don't have a [good] prior history with. I get the same way (though, oddly enough, RPing is the exception to this; writing with new people, getting to see different kinds of ideas... that's kinda my jam); this might be harsh, and a callout on somepeople, but if you're the kind of person that genernally prefers to write within their established circles, keep your GMing to that. I realize I may be alone in this perspective; it is what it is. 

Otherwise, it's helpful for GMs to be transparent about what standards they will be holding people to. Most do so in terms of post length, and general RPing rules. But being equally clear about things like how collaboration will be enaged. [i]Most[/i] RPs I've been in have asked little in the way of prior collaboration before engaing in IC interactions. A few have expected more advance planning. The latter cases tended to not make that part clear until after a blunder had already been made. Overall, just managing the expectations of those that would be interested, especially if you are accepcting new-to-you faces (who may not quite know how you roll). 
[*]Kinda something I've been gnawing on for a minute: as someone who has known having either myself or character rejected upon application, I always find myself wanting feedback from the GM(s). Was it a character issue? Do I, as a player, simply not check enough of the boxes off? Yes, this is just a hobby, but, as with any hobby, it's fun to get better at as time goes along. While it tends to be rare outside of Advanced RPs to outright not make the cut, I'd love to see more GMs giving actionable feedback in those situations. Unless, of course, the reason for not accepting someone is simply because the RP has hit its maximum capacity.[/list]