Has anyone gone through most of the other quests before starting the main quest? How do you avoid starting it and are there any drawbacks to waiting until last to start the main story?

don't you have the same question in the help section?
here's the same answer: To completely avoid it I think you must not talk to the guy in riverwood who will send you to whiterun's jarl.

Just ignore bleak falls burrow until you are ready to start the main quest. But there is some stuff tied to the hip of the main quest so you'll only get so far in other areas of the game, which I can't stand personally, but I didn't make it.

I played two characters, for a total of over 100 hours, without beginning the main quest. I just turned left or right as soon as I got out of the tutorial dungeon.

I have been playing pretty much since the game release, pretty much every day, and I have not done the main quest.

I got as far as being told to go up the 7000 steps, but I haven't yet. This is with two different characters.

Mostly I do side quests or quests relevant to my current character. For example, my current character is a Breton Mage. She does not believe that she is the Dragonborne because she is not a Nord. And she is focusing on her studies at the Winterhold College, and getting to know Skyrim (ie exploring and side quests).

In TES IV: Oblivion, I didn't do the main quest for a very long time (well over a year as I recall). Same with Fallout 3 and Morrowind. So this is typical for me.

I'm only up to the second quest of act II (according to this http://elderscrolls.wikia.com/wiki/Main_Quest_%28Skyrim%29), and so far I've been roaming all around the map.

Still according to this same Wikia, the only drawback I'd see is that one quest not becoming available for you if you've already completed the "choose Stormcloak or Empire" line of sidequests...

Other than that, beats me.

Haven't touched it after ~150 hours.

There is an advantage to doing the very beginning stages of the MQ. The Greybeards will give you a few shouts and tell you where the rest are located.

I just got my Orc to level 81 without ever talking to the anyone in river wood (quest related). I actually am starting to prefer it without dragons, and I can still find dragon armor as loot.

What? You actually have no random dragon attacks while playing the game? That ruins the game in my opinion.

You can do Bleak Falls Barrow, get the dragonstone, and deliver it in Whiterun. That will grant you right to buy house in Whiterun. So far so good, done this in most of my games. I expect the dragons to be "released" once you get near the western guard tower where you fight your fight dragon. Only drawback afaik is no access to shouts, obviously.

I haven't completed the main quest yet, nor most of the other quest lines.

No civil war either, you have to deal withthe dragon problem in Whiterun first.