To some extent human beings, no matter what type, have to plan.
Let's just get the stupid thing out of the way first. Everywhere I've looked, all the forums I've been on, identify L as an INTP. I'm prone to believe several different sources that come upon the same conclusion more correct.
L plans some things, but during his arch he ends up doing a lot of random crap his successor Near doesn't.
Let's remember what I just said, planning is something that must be done to a certain extent. You can't simply say that L plans, therefore he's a P type. Everyone plans, if you didn't plan anything you couldn't easily survive. In the case of that point though, L has Watari and Watari does a lot to help L. He couldn't survive without the assistance of other people.
That's extreme, no real human being is like that.
But let's look at L's behavior from the get go. He chooses to investigate the sudden unexplained deaths of inmates because he wants to. There's no schedule for L, he doesn't have a list of projects to work on. He just takes whatever interests him.
Let's contrast that with Near and Mello. Mello doesn't go with what was previously planned, he walks out and decides to do things his own way. Near could've done that as well, but instead he sticks to the previous plan and with the people he has (which is hugely beneficial for him anyhow).
In the past L has had to command police departments, he can't afford to be careless with people's lives. For the sake of directing a team he has to have direction. Some of that is obviously learned, though experience.
L's Planning (investigation wise) and Decision making are spontaneous. He isn't thinking that far ahead, that's why he gets himself into bad situations. He's going on interest and a gut feeling.
L doesn't plan to go to Light's school, that's something that comes after the hidden camera investigation. He springs that on the police department after meeting with his team, which is something that he's never done before (it's spontaneous and supposedly a sign that he's serious about the case). He does tell them what he's thinking of doing and has the belts for them. But that might've been some of Watari's doing, seeing as he's the one responsible for the belts.
L merely has a hunch about Light when he takes the entrance exam and tells Light, (in all places- at the entrance ceremony) that he is indeed L. Despite the fact that this SCENE was planned. L doesn't think too far beyond that, though he may have hoped for Light to join them. Then again, I don't think he dwelt on that, he realized, as is stated in the anime and manga, that he'd put Light in a corner by telling him that he's L (if Light were Kira).
L is not like Near, where he makes complicated plans and has back up plans. He thinks to put a road block where he can, maybe so he doesn't have to plan too far into the future. He likes working spontaneously.
I think it's when they're getting into the Second Kira scenario that, suddenly, L realizes they need more TVs.
There isn't a hand book on how to run your state for Wammy's kids, but Near already knows to have more screens than any person could watch, except for him. I don't know if this was an ability that only Near had or if L did too. I thought maybe L did as well... That's besides the point. Near's station is already planned out and he doesn't change it, aside from his various toys (and the situations he's forced into).
There's another instance where L acts spontaneously that I can recall off the top of my head. The situation with Matsuda. They throw together this plan for Matsuda to "die". Which I think was more like L thinking "I want him to die". Anyway, L finds himself having to play the part of a medic and he express his distaste for having to do so. This actually ends up working out well, because Misa is smarter than they thought (or has her moments).
Where L really *beeps* up is with Rem.
Near is in a shittier situation with Mello on one side and the Japanese task force on the other. When he's under siege he has a back up plan. Dump a million dollars on the greedy bastards and escape in riot gear while everyone else is confused (this probably means he already knows where they're relocating to as well). He also knows, before the Japanese Police hand over the notebook, that he's screwed if they hand it over. He already knew what was going to happen. Near plans ahead as far as he can.
But looking at behavior only does half the justice.
The important thinking difference with INTJs and INTPs is simply whether or not they're willing to work off of conclusions.
L experiments. Most of his method is experimentation. Wait and see, then do. He makes short term plans.
Near and Light's method is planning. They make conclusions, then plan in the case of that problem.
An INTP experiments to find a good way and may not dismiss other ways for a long time.
An INTJ can't think without making conclusions. They find the best way and then they do it.
In practice it makes INTJs the winners at chess games. INTJs are far ahead of the game, unless an INTP knows chess really well and what amateur moves to avoid.
Light is probably an NTJ. He plays chess with L. That's how their story works out.
Light makes plans and L moves accordingly. It's all the more important to note that L works off of hunches. While Light works off of conclusions.