What to look for in a bike
 

I'm going to make the move up from my first mountain bike Giant XTC 3.5 to a better bike. I want it for general racing and spins. I dont have much of a technical knowledge of bikes and was wondering what to look out for moving up the ladder?

What I'd like is a bike that feels better, has more sensitive breaks and is a bit more reliable and smooth with gear changes. Any things to watch out for appreciated.

This is kind of vague- what's your budget? FS or hardtail? More detail would be nice.

Ride as many different bikes as you can. Every bike fits differently, and alot of it is personal preference.

Ask other people that you ride with what they like about there rig. Full suspension or hard-tail. Maybe even 29er.

If your looking for a FS rig, research through all the different designs, 4-bar, Virtual pivot, Maestro, etc. See which one fits your riding style best.
And, when looking for a bike, compare, compare, compare.
Get the geometry and compare
Get the component list and compare

Just do you research, and ride as many different styles as you can

I'd be thinking about a hardtail budget about 3,000. I need to know what components and parts I should be looking at. What are the important factors in choosing a bike.

3000 what? US Dollars, Euros, AUS Dollars etc.

At $3000 your not likely to find junk. The thing to watch out for is fit. Everything else is secondary to having a bike that fits you and feels good to ride. Stop asking silly questions and ride some bikes.
If you'd like some answers gere are things I usually watch out for when dropping several thousand whatevers on a bike: missing wheels, backwards fork, one side of the handlebar missing.

And missing tubes on the frames. You can never be too careful.

But honestly, budget $2500ish for the bike and get some good accessories if you don't have them- nice clipless pedals, shoes, clothes, helmet, sunglasses, etc. You'll notice a bigger difference spending more on those than $500 more on a bike.