Well it depends on what you mean by obsidian. If you're trying to get a nether portal, then all you need is wood>wooden pick>stone>stone pick&furnace>iron>bucket.
If you're attempting to get an enchanting table, and hence need the blocks in item form? Yeah. You get to go through the oh-so-strenuous "tech gate" of: Get one log, make crafting bench, get two more logs, make more planks, make sticks with two of those planks, make a wooden pickaxe, mine into the ground, make a cobblestone pick with the spare two sticks in your inventory, make a furnace, find and mine 3-6 iron, find another log, make more planks, make more sticks, make a pickaxe (and a bucket if you aren't going to aquaduct water to lava), find diamonds, mine 3, make a pickaxe, mine obsidian.
If I recall the steps correctly, that's easier to describe than making a tech tier 1 generator in IC, not even IC2 or IC2-Exp, if more time consuming due to mining requirements. So why is an enchanting table considered to be higher tech, or later game than an IndustrialCraft generator? Because vanilla's "tiers" are based on the flawed assumption that rarity = difficulty. It doesn't, it just means (statistically speaking) more time. For some people, it will take less time to build an iron golem farm (sans villagers), or a castle, or a cathedral, or pretty much any other megaproject, than it would to make and fully stock an enchanting room. Because they can't find cows, or reeds, or diamonds are extremely elusive for them.
Whereas Rotarycraft involves you finding redstone, iron, a carbon source (which one for the life of me I can't remember right now), and creepers to get started (yes, I dropped out the obvious steps of making the various pickaxes, and the furnace, and the stone bricks... Stuff that any game which isn't peaceful or creative will have you do). I have yet to meet any minecraft player that knows the simplest things about MC, like redstone is deep in the earth and mobs spawn in the dark, who could not have a piece of HSLA steel by their first or second MC dawn if they decided to dedicate themselves to doing that. One time I spent an entire MC month trying to find the reeds and cows needed to make books.
If you take it to the extreme and want to be particularly focused on the exact definitions? Any mod that doesn't rely 100% upon things you can harvest completely by hand is "tech gated" or has a "tech tier". But just because you will probably manage to not find lava or a cave underneath you, doesn't make digging the block you're standing on a good idea. What is technically true, or even probable isn't always the point, especially when one is talking in broad terms.
Assuming that you have 100% completed vanilla's "tech" You should be perfectly able to build a MFE from any industrialcraft version without crafting a single machine first. The same goes for every single machine in MFR except for the Laser Drill Precharger (you need a slaughterhouse for pink slime first).
Thaumcraft requires a wand, a bookshelf, a research table, ink and scribing tools, and a thaumometer to unlock (nearly) the entire tech tree; only the special unlocks aren't obtainable after you've Scanned All The Things.
Botania only has pre-botania, the petal apothecary, the pure daisy and a mana generator to get living rock/wood, the altar, and then IIRC you're at the top "tier" of the mod; you can craft anything.
IC2 (pre-liquid UU), got Compressor and Macerator? You can make anything else in the mod.
Buildcraft requires a single engine, an assembly table, and a laser to fully unlock the mod.
The same basic progression happens for nearly every mod out there. There generally isn't any tier you can't reach in less than 6 steps, assuming you've gotten a diamond pickaxe.
Some exceptions are Blood Magic, Gregtech, and RotaryCraft, (probably ReactorCraft and Electricraft too). There are large portions of those mods that simply cannot be obtained without having done work in the previous tier, often multiple things. Try making a fusion reactor from Gregtech with the completely raw unprocessed materials in front of you. How about a Bedrock Breaker, a Master Blood Orb, or a Tokamak reactor? You're going to have to progress up the tech tiers for those mods to do so.
The way I look at it is this: Assume you have a fully stocked vanilla base and literally infinite raw worldgen resources. Can you make every item in the mod without having use or place any mod items? Then there are no tiers. If you only need a single mod item/block to get all the tech, it's got 1 tier. If you need two items/blocks and one of them requires another mod item/block to be made? 2 tiers.
IC2 Exp has 4 tiers, because liquid UU is made in a T3 machine, in other words making it requires another industrialcraft machine to be made first. In this case, a macerator, a compressor, and a power generator of some sort. Those machines cannot be made without the hammer and cutters , which are vanilla crafting recipes.
I'd list off the Gregtech or RotaryCraft tiers... but I don't know them well enough, and the research on a wiki or in-game would take significantly more time than I care to put into this post, because at this point, my point should be clear.
Edit: It might look as though there are a few mistakes in my early tech tree examples, like forgetting about the IC2-EXP hammer and cutter requirement for making an MFE, but I was discounting tools when I first started this post and I've already re-written this entire wall of text too many times to want to do it again.