What other commonly observed unspoken rules? Game etiquette

People will flame you (hopefully only occasionally)

That's part of the DotA experience, and especially so if you are new. It's inevitable but not as bad IMO as it used to be when there weren't matchmaking algorithms (WC3 DotA, where complete newbies could end up on teams with people who had been playing for years). The Valve matchmaking algorithms are actually pretty good so you should end up with people who, if not new, at least play at a similar level.

Really, hate is transitory. This or that player might hate you for one game, then you part ways and never get paired again. At least the odds for it to happen are very low, and even if they're in the same match they could be in the opposing team
Do not take it to the heart is someone is flaming, calling you noob, etc. You are a noob, that's no shame, if people don't want knobs in their game they should start making friends and look for games as a party. Oh yeah .. flamers don't make friends. Also good players don't end up paired with beginners.

Ask questions if you have them and if people seem friendly and willing to give it.

If you have to pause, give a reason for it in all chat so everyone knows. Sometimes your pause will be respected, sometimes not. I find people are usually courteous enough if it's not for counterpicking in All Pick or if you haven't already provoked them.

• I like having >Well played! on my chat wheel for when an ally gets first blood or makes a good play. I like building up camaraderie before/if shit hits the fan later.

You can ctrl-click (or maybe it's alt-click, my fingers know but I don't remember right now) your abilities and items to tell teammates they are on cooldown

This is important for big teamfight ultimates like Black Hole, Ravage, Haunt, etc. It will tell your teammates how long the spell is on cooldown. Alternatively, it will automatically tell them if you don't have enough mana for it (blued out icon). If it's a team-based item it may instead say "Gather here for [Mekansm/ Smoke]," as well.

Apparently you can now do this for items in enemy inventories as well (which I didn't know until today)

It tells your teammates that "Enemy X has Y item." Usually only important for Blink Dagger or Black King Bar or Refresher Orb or other critical teamfight items.

As a corollary to the last paragraph, the little diamond under each hero icon on the top bar shows whether that hero's ultimate is ready

If you hover over an ally's it shows you how long until that ult is ready. Not really an etiquette thing but a handy play tip nonetheless.

Most of the "etiquette" comes down to expectations management

In noob games, holding your lane and trying to help your team will be extra sufficient. Hugging your tower is preferable to risking a dive and feeding a couple hundred gold to the opposing hero. This is actually a good reason to practice denying your own creeps, so you can drag the creep line close to your tower.

As you grow, you'll find more is expected of you, but people will actually tell you that. If you pick a support, they'll yell "wards" 3 minutes in, don't worry you'll get no chance to forget your role. If they don't say what they expect you to do, they have no right to complain either.

Courier hogging has been mentioned as a no-no

You also need to pay special attention to warding if you're against Riki or Techies; bottom line, if you lose because your support didn't ward, you still lost. Assigning blame post-match doesn't really help.

There are some heroes that have abilities detrimental to team if used incorrectly

Let's say the opposing team has attacked your mid, you outnumber them and they start running back to the river. An ill-timed Chronosphere  from Faceless Void could catch none of them, AND will stop your team from giving chase. Don't be that Faceless Void. Similarly with Earthshaker's ability, accidentally trapping a teammate in Tusk's ice wall, etc. Thankfully the list is short.

As you keep playing, you'll learn to be more aware of heroes and the order in which they "come online". You'll want to play extra safe against Viper  but delay & harass Riki as much as possible, then be extra wary of Riki when he gets level 6 (his invisibility). You'll learn to be wary of Witch Doctor when he gets his ulti, generally a lot of ulties can be combined to gank you. But this is actually too much to keep in mind if you haven't actually played your first multiplayer game, so I'mma stop here.

Don't start a sentence with "Why did you" or "Why are you" or anything similar.

Intentionally typing anything about or to your allies in allchat is almost always bad mannered. Blame-shifting is stupid. You'll never get better if you don't see your own flaws instead of everyone else's.

The ping is to be used as a tool to alert allies of something going on in the map

Pinging on an allied hero once to, say, alert them of immediate danger is fine. You should probably never have need to ping more than once per second maximum.

A good way to nudge a teammate to do something without seeming like "That controlling jerk" is to say things like "Would you consider buying this item?" or "Would you consider doing this action?" and then giving a quick reason behind your request. Telling someone "Go do this" might annoy them even if there was no rude meaning behind it.

• If it looks like your team is going to win, it's bad-mannered to call GG before the game is over or before someone on the enemy team calls it.
• It's rude to taunt enemies after you win - they already just had a rough game, no need to rub it in.
• It's pretty common for the winning team to kill the enemy team in the enemies' fountain before finishing the game. Don't draw the game out way longer than it needs to be with this. Kill the enemy team once or twice but then go for the ancient.
• Try to have at least 1 ward on a rune spot throughout the game if possible. This is just something nice for whoever on the team is carrying the bottle. To add to this - if you're carrying and nobody's buying wards and you're dying because of it, consider the fact that you will lose much more exp and money by dying than you will by just buying wards and placing them. Sucks but it happens.
• Be supportive to teammates having a bad game and encouraging to the ones doing things right. Negativity never won any games, and it won't make you feel any better either way.
• Give your mid a tango if you're supporting. Every bit of help towards your mid is important.
• Learn to gank mid. It's way easier for the mid to win, and then start heading out to help the sidelanes, if you get an early mid gank off. Use smoke.