I kind of want to start roleplaying again, but looking at these large established roleplays is a bit daunting. The problem is that I don't really want to read up on these roleplays and would rather just jump in. I'm not sure if that sort of thing is frowned upon here, though? What do you guys think? (I guess I could just PM the OP and ask for a summary, but I feel that would be kind of rude? I dunno.)

Note: I don't really want to do one-on-one roleplays because I feel that it would be pretty awkward.

It's not all that common, at least not that I've noticed, but there are a handful of threads like that. Typically, they say something about just jumping in. Here are two examples:
Dead Horse Tavern and Inn
Svetopia

Also, if you check some of the Request forum for threads marked with the green "Group" tag, some of them are relatively new. Newer ones typically have only a handful of posts. They can be moved down kind of quickly, though, so it might be best to check the second page as well. :3

As far as I know, it's generally not looked down upon to ask for a summary if there are an excessive amount of posts. :P If there's only around twenty or so, I personally think it's better to read them.

Thanks, that was really helpful! I wasn't really aware of the request forum or the forum etiquette over here.

No problem. :D I was glad to be of service.