I play a lot of ranked and floating between 1450-1550 elo. I would say almost every (if not, every game) there is someone committing one of the reportable offenses. The ones which I find most common in order are:

1. Negative attitude: A whole lot of, gg its over, so and so fed, etc. Just general talking down the team and being very discouraging/raging while trying to get everyone to give up and surrender. Or often trying to find a scapegoat to blame the loss on all game.
2. Verbal abuse: A lot of harassment directed from one player toward others.
3. Offensive language: Enough said, this is super common.

Unskilled player. Why does this button even exist? I don't think you should get punished for having a bad game, or just being bad at the game.

Negative attitude is the one thing I was never too sure about when reporting. The description is very vague, this seems like there is someone almost every game with a negative attitude.

The fact that almost every game I see someone who I truly think commits one of those offenses makes me think that tribunal isn't very effective. Furthermore, I think this sort of negativity has become so common online (especially in league) that people generally accept this level of negativity and frequently don't report these things (especially for negative attitude). Thoughts?

Yeah, I'm thinking if you're reporting every game you're probably being too sensitive or seeing things that aren't there. I suppose I could be wrong, though.

In particular, remember that you're supposed to be reporting players that intentionally make the game worse for everyone else, not just anyone who fulfills the letter of a reportable offense. It's fairly easy to claim that you've lost or say you should surrender without being overly negative about it, for instance.

Read unskilled player's tooltip. It isn't a punishable offense. As for why it exists, it's either the stated purpose of refining the reported player's elo, or my personal theory of being a magnet for ragereporting, so it doesn't get mixed in with genuine issues quite so often.

As for your general depression about Tribunal effectiveness and online mentality, not really. Especially in a free game like this, it's hard to block any and all bad behavior; the best you can do is try to mitigate it, which the Tribunal appears to do. Online stuff has also always been like this, so it's certainly not a new problem. Some lack of reporting may be apathy or acceptance (neither of which is bad, mind you, so long as they're not unhappy and still not reporting), but I still suspect you're being oversensitive in a lot of cases.

Trying to get the topic back on track after the relevant last comment....
According to the system in Riot, over- reporting is punisheable. Been there when the massive ban for abusing the report system was implemented. You don't get a reform card for that, it is a separated ban...
I don't think - still up to date- that I falsely report as in accusing someone of feeding when they are clearly having a terrible game for example.. But I did learn that reporting at least one person each game shows a lack of common sense and filtering that I totally understand because in the heat of the moment everything is more annoying that what it really is. And it can backfire. Not only you'll get processed for spamming reports, but also your own reporting weight will be lowered, so each time you Do report for something that really went wrong, and ruined everybody's game your report will be useless.
Unskilled is not a report for the tribunal, because it serves as a filter to matchmaking.. if a person gets a lot of unskilled reports, the system will match them with lower levels.
I don't consider it WRONG , but for the punishment system it can be checked as spamming.

Can't handle all the relevancy D:

Ranked is pretty rough. I only do normals now, so it's rare that I have to report someone, but that might not be too unusual for ranked games.

I will state upfront I am a harsher judge (I do read the cases though)

I find myself reporting in 20 - 25% of ranked games and about 10% of normals, and pretty much never in Dominion.

I guess I don't really care if someone is negative, as long as they aren't being a jerk to another person, and continue to play their best (People of say we will lose, but still play, if you say gg then hang around in base or troll then that's different)

banned typed in chat during a game

Even if you report a player every game, that only means about 10% of summoner could improve their play. That doesn't sound like an unreasonable number to me. Although I think Riot has stated the percentage of players they consider "toxic" is much lower than that number.