Timezones
I also regularly see wildly inaccurate date information and I'm in US Central time.. so I'm not convinced the issue(s) is/are tied to timezones.
Leaderboards
I don't know who specifically you're referring to but I certainly don't believe "half of the users" just have all the lessons marked complete. There are definitely some that aren't as active in the forums as they once were (myself included to a certain degree), but I don't believe it's nearly as skewed as you believe it is.
Locking of old threads
Not sure if outright locking and prevention of further discussion is quite the right approach, but maybe some kind of "Are you sure you want to revive this thread" kind of message reminding the user that it's rather old and likely they're better off starting a new thread instead would be beneficial. It is certainly annoying to get a notification of activity on a thread you participated in years ago, often just because someone decides to spam their new package to a bunch of barely relevant old threads and so forth.
Prefixing thread titles
Some manner of indicating the relevant laravel version would be useful, but I don't think the assumption can be made that it would be whatever was the current version at the time of posting, and my experience has shown that users generally can't be fully relied upon to provide all the relevant information when seeking assistance. Maybe the inclusion of a "Select the version of laravel" widget on the post creation form would help, but it may likely just get ignored, or wrongly assigned too.. no real silver bullets here.
Thread suggestions upon thread creation
This would be an interesting feature.. Would require some sort of recommendation engine, based on.. what? The entered post title? That leads to another major issue with user-generated content, fairly worthless titles :) But interesting feature, if that nut can be cracked in an intelligent manner.
Forum Titles
Don't the existing badges already more or less accomplish this?
Caching issues
Not sure I've experienced the behavior you mention here, but even if I had I'm not overly bothered by a bit of delay due to caching, the site feels speedy, and it's not ever been advertised as real-time communication, so if caching keeps the site speedy, and operating costs down for Jeffrey, I'm not personally bothered by any caching delays.. but that's just one mans' opinion :)