First off, welcome to Anime & Manga SE. Most of these issues you have raised are not specific to this site, and are rather general to Stack Exchange. I'll address these in turn.
I can't tag one of my most recent questions as "07-ghost" because I don't have 150 reputation...
This is a built-in limit which we have no control over here. Personally, I'd like to see it removed, or at least for new users to be able to suggest tags on their posts. But this is a rather small site on the Stack Exchange network, and the problem is relatively uncommon on most sites, so we have to deal with it.
For now, our suggested solution is to use the retag tag and specify in your question the intended series. A high-rep user will edit the correct tag in. I wish this were more intuitive, but there's really not much we can do about it.
I can't comment on questions because I don't have 50 reputation...
You can comment on your own questions and answers to those, but you're right that you can't comment elsewhere. That again is Stack Exchange-wide, not specific to this site.
The reason for this is that, unlike new questions and answers, new comments aren't likely to be seen, so it's difficult to identify spam comments. I don't claim that you're a spammer, but the system doesn't quite trust you yet. In any case, the core of the site is questions and answers, and comments are mostly temporary and not worth any rep, so you honestly aren't missing much.
I found a question I was pretty sure I could answer (Help in remembering an anime!) but I can't answer it because you guys voted it "closed"
Here, you've stumbled upon an unfortunate case. identification-request is somewhat controversial among members of this site. Almost everyone agrees that good identification questions should be allowed. But also, most people agree that a lot of incoming identification questions are low quality and don't stand any realistic chances of being answered.
Because of the volume of incoming questions, we can no longer afford to treat each one individually, so we have some minimum standards for new identification questions. It's worth noting that these guidelines are somewhat controversial, but for now they're the accepted policy. If the question doesn't meet those, we put it on hold until the poster improves it to meet those. In some cases, the question might be answerable even when it doesn't meet the criteria there, but to keep things consistent the rule is still enforced. So, with that in mind, you unfortunately won't be able to answer closed ID questions like that unless the asker comes back and improves it, or you manage to convince other users that it shouldn't have been closed.
In this case a moderator reopened the question you pointed out, so you should be able to answer it now.
My attempts to edit it all get rejected because "This edit was intended to address the author of the post and makes no sense as an edit. It should have been written as a comment or an answer." (Fair enough, but editing the answer in is literally my only option.)
I'll assume you are unable to post an answer yourself because the question you're looking at is closed or on-hold. That's the only reason a user like you with more than 10 reputation wouldn't be able to post an answer on an undeleted question. In that case, the users here have decided that it violates one or more of our policies, which you can read in the help center. Usually there will be one or more comments on the question saying what is wrong, and a banner.
In some cases, the question can be improved to be reopened. If it's an identification question, that improvement usually has to come from the asker, but for anything else, you can suggest edits yourself. If your suggestions don't change too much and fix the problem, they'll probably be accepted, and the question will be put in a queue to be judged for reopening. Once it is reopened, you can add your own answer
Alternatively, if you think nothing is wrong, you can flag the question for reopening immediately, or you can come here on meta and ask why the question was closed. Compared to other SE sites, we're actually pretty lax in terms of closing questions, and we'll usually reopen a question if a good argument is made for it.
The one thing that doesn't really make sense to do is to edit existing answers on the question, especially if you're trying to add a bunch of your own thoughts. If the question is closed, that means we've identified that it has problems. If nothing is done about those, we'll probably unfortunately eventually have to delete it. Rather than trying to edit your own answer into another one, you should try to fix the problems of the question first and foremost, and then once that's fixed, post your own answer. That probably sounds like a lot of work for just one question, and it won't happen in a matter of minutes, but questions here are intended to stick around for a long time and in the grand scheme of things our approach eventually gets things right.
I don't even know if the above issues are because of StackExchange or this specific community...
This is probably because you're still a new user and unfamiliar with the system used here. Try checking out our tour page to get a general idea of what the site is for, and the help center for more details. If you still have questions, this is the place to ask.
I realize this system is rather unfamiliar to new users, especially those who are used to the more lax policies on fora and social networks. We do judge the quality of posts somewhat strictly, and at the low end a lot of things are restricted to prevent spammers from having their way with the site. I think you've confused that unfamiliarity with us being unwelcoming, but that definitely isn't intended. The low-end limits aren't intended to stick around for very long. You've only been on the site for 2 days, and once you're a bit more familiar with the system and the site and you've earned a bit more rep, those will disappear.
If you're looking for ways to get reputation quickly, I'd suggest checking out the unanswered questions. There's a lot of them, and you can probably find a couple that you can answer. You'll probably have more success if you spend a lot of time writing a few well-researched answers to good questions rather than just trying to get an answer on every question you can. Our users here value quality a lot, and are much more likely to upvote an answer which goes above and beyond.
It's also worth noting that if you know a lot about a series that people haven't already asked about, you're allowed (even encouraged) to answer your own question, which can be a great way to gain a lot of reputation if you do a good job. Finally, while there aren't any right now, you should keep an eye out for bountied questions. If you can answer one of these well, it's an easy way to gain a lot of reputation.