What is the most crucial element of a mountain bike?

I'm kind of a big guy; I dont do "hardcore" trails, only very light, little to no rocks/tree roots etc. If I were to modify my bike over time, where should I start? Something cheap.

TIRES, the best designed bike in the world is worthless with a poor tire selection. While tires are not a real cheap upgrade a bad or poor performing set will make riding an unpleasant event.

Hardly anything as far as upgrades is "cheap". However, if I were you I'd start with tires. Do a bit of research and see what design is best for the conditions you ride in. Next, I would go with cables. I don't know what kind of bike you have but I'd replace cables with stainless steel versions and housings with compressionless stuff. Amazing what a really good cable can do to benefit shifting and braking.

Once you do that, it is a toss up between drivetrain and wheels. Upgrading either of these can get pretty pricey but tend to be worth the cost.

After wheels and drivetrain you'd probably do well to consider a new, better bike.

I see in amazon buyer guide
Amazon.com Bicycle Buying Guide

Fit is crucial for comfort, control, and proper power and endurance on a bike. You can see information in myblog astore cheap bike ,information ,price ,spect. Thank you .

All good answers...

What's your goal in upgrading your bike?

Trying to shed weight? Improve braking, traction, climbing, trail manners, what?

I'll echo everyone's sentiments on tires. That's the cheapest, best improvement you can make.

Other simple mods, a good saddle. A comfortable seat that doesn't crush the gooch is a plus.

I had a guy in a bike shop tell me long ago that the piece of the equation that needed most improvement was ME.

Whether it was losing a couple pounds, or working on riding my skills... that advice makes some sense. Riding along with a more experienced rider can help more than hundreds of dollars in a shop...

For big guys I always reccomend some tough wheels and hubs. A stiff fork after that. If you dont have clipless pedals get those, they change everything. After that just replace stuff that brakes with better stuff.