Don't return the phone, there's nothing wrong with it.
I've had 4 Galaxy S2's pass through my ownership so I think I'm pretty well equipped to say whats normal for these phones and whats not.
Wifi reception IS poor. My old HTC Desire and my girlfriends HTC Desire both get MUCH better range on the wifi than the Galaxy S2. The range on all the Galaxy S2's I've had has been the same - poor. This is a common complaint about the G2. Anyone who says they don't have this problem either has no idea what good wifi range is, or is just lying out of their ass.
The screen does indeed have a blueish tint on it. There's a thread about it on these forums. Nothing we can do about that. Again, another common issue to all G2's.
Also the some other issues you have mentioned. Touch screen sensitivity is on the low side, you can tap the screen lightly and quickly enough so that it doesn't register an input. This is impossible to do on a HTC Desire.
Also the pixel response time is said to be 0.001ms by Samsung, but this is a load of crap. It works out closer to 6-8ms, as evident when scrolling black text on a white background (or vice versa) the text will blur slightly.
The GPS is pretty poor at the moment, hopefully it improves with updates, majority of people are complaining about this, myself included. None of the 4 G2's I had could get a quick lock on GPS (they take flipping ages). The GPS is REALLY hit and miss at the moment.
- Random Reboots
This is a REALLY REALLY common issue. It's been reported everywhere. However, depending on the frequency of the reboots would determine whether or not I'd class it as faulty. Of all the G2's I've had, NONE of them have randomly rebooted. However if yours has rebooted like once or twice over the week, I'd count that as normal, happens on a few android phones. But if it does it anymore than that, get it replaced.
- Poor Sound Output
This is completely subjective. But I have an expensive amp and speaker setup that I use on my PC which I've got used to so I'm really picky about sound quality and yes I agree, the sound output is abit naff. But it's not terrible by any means. I can live with it.