Despite laning being the most constant and predictable part of each match, I see a lot of mistakes that can probably be blamed on how experienced or focused players are in the skill area I'm playing right now. (which I'm guessing is with other fellow noobs) I'm guilty of screwing up in the laning phase too, and it's something I'm trying to correct. Below are a lot of the more common things I notice players doing, along with my own shortcomings.
TL;DR version: The Lane-phase is important. Focus on farming, smart play, and being a good lane partner so you can have a successful match.
Auto-Attacking Constantly
In just about every "New Player" guide that exists out there, there's a large portion explaining how you shouldn't auto-attack in the laning phase. It pushes the lane closer to the enemy tower making it harder to set up ganks. You can't guarantee last-hits when auto attacking. You are trapped in attack animation making it easier for the other team to harass you and the list goes on. The only reason I think people do this is because no one has told them, or they've never read a guide.
Taking last hits from your carry
If you're in a lane with a hard carry, let them farm. Let them get their last hits and get big for the end game. When I'm in a lane with a carry I focus on harass and denies. It let's me contribute to the success of the lane while making sure I let my carry get the farm he needs. So just remember, if you're playing support, or a less farm-needy hero, let your carry get the farm.
Going for blood instead of last hits
A hero kill is worth around 250 gold. First blood is worth around 400. That's all good. And yes, killing a hero helps your lane a ton. However, getting 5 last hits is worth around 220 gold. Getting 10 last hits is worth around 400. So you decide. If you're ignoring wave after wave of creep hoping to kill off your opponent, you're doing yourself more harm than good.
Chasing after a hero close to death instead of letting them go
In the ace "Welcome To DoTA, You Suck" guide, there is a very helpful piece of advice. "Your goal is to KILL or ALMOST KILL enemy heroes." Almost killing a hero and forcing them to run back to spawn is just about as beneficial as you killing them. You get to farm with ease. They are denied an opportunity for gold, and denied ALL XP while they run back. It's a lot better IMHO to let someone run back to base than to go chasing after them and possibly die to a returning teammate, a gank because you've pushed to far, or to the tower while you dive with blood lust. So don't be sad when someone survives with 30 HP. They're running back to base while you pick up five last hits and soak up XP, and when they get back at full health you're two levels above them with a new item or two.
Leaving the lane and not leaving
Knowing when to leave the lane, and when not to is important. If your character doesn't have the tool set to help gank mid at level 4...don't leave the lane to walk to mid to do nothing. Don't leave to go the secret shop just because you hit 875 gold and you can buy that first regen item for your build. Every time you leave the lane you lose out on XP and farm. Make sure you leave for a good reason, and buy a TP before you go so you can come back quickly.
At the same time, if four people are pushing your tower and you're the only one there, it's a good time to leave. I've constantly made the mistake of thinking that me hugging a tower against a wave of creeps and 4 heroes is some how going to make them turn back. Yeah. I die. All the time. I still do it. I'm trying to fix it.
Being unaware of the the capabilities of other heroes
This plagues us new players because we just don't know the roster yet. It's why I encourage noobies to play on random for a while and try out heroes they've never played because it's the best way to learn what skills another hero has. This way, when you face them in lane you know what to expect.
Queen of Pain and 
Sniper for instance, have huge finisher potential at level 6. 
Zeus and 
Lina and 
Tinker can kill you in one combo early on. 
Riki (a stupidly popular pick in lower ranked Pub games) goes perma-invisible at level 6.I ate a sniper ult with low health for an instant kill during one of my first games.
Conversely, i ate a
Sniper in one combo with Queen of Pain a week later. Which was very rewarding.Conclusion
The lane phase to me is the most important and the part of the game that is going to set up a team for success or failure for the rest of the match. Focusing on a successful laning phase, is a good place for new players to start and part of my game I am constantly trying to improve.
learning to farm (last hit), which in turn teaches basic mechanics - leveling, skills, kills, pushing etc
learning heroes - chances are you will have a handful of favorite heroes who your happy to use a few times everyday. To experimenting with new heroes (this is the stage where most noobs go from playing a carry, thinking they can farm all day - to playing a support and realising how important you are and how much more fun, becuase you're not sitting in the lane doing piss all, but farm)
learning the map - fog spots, juking, neutrals, ancients, roshan, warding, day/night etc
Learning the timing - when to ulti, when to flight, when to fight, who to initiate, target, when to tp, when to push etc
learning the clock - and probably the most difficult - knowing the wards are gone on the 6 minute rune, and it just turned to night. Any mid hero that doens't gank then should be shot! support heroes getting sentries at 7 minutes to destroy the freshly bought and planted wards, leaving the team blind for about 3 minutes! rosh timer, pulling creeps - all that lovely stuff!
dota is an awesome game, I've played for so long, and still learn stuff all the time!!