More than any other tool in the kit, a reliable lever makes changing a tire easier, especially if you have road bike tires, which are difficult to remove. However, throughout testing, levers seemed to be the one item more prone to failure and poor design than anything else. For example, I found a random orange lever floating around my basement and I threw it in the test pool for fun—it seemed solid enough … until I began prying at the rim of a tire. It bent directly in half, slowly and smoothly, like taffy, and then was boomerang shaped forevermore.
It’s both surprising, and not, if you think about it. The way I’ve usually seen it happen is first the rear tire goes flat, then the front one. My theory is that both tires went through the same pile of glass/thorns/metal bits and they both picked up junk. the reason for the flat occurring on the back first is because there’s more weight on that tire, so the offending article works its way through the tread more quickly.
For the budding mechanic:  A good pocket sized repair guide.   One good option is the “Pocket Guide to Emergency Bicycle Repair”  by Ron Cordes and Eric Grove.   It’s only 3.5×4.5 inches and a half inch thick.   I don’t carry this myself, but for someone who’s new to this whole bike repair business, it’s really helpful to have. A future blog post will cover this book in detail.

As you mentioned, it is surprising how often you can get multiple flat tires on while riding, particularly on longer sprints. The tip about bringing a patch kit in your bike repair kit can be incredibly helpful if you’ve already had one flat tire. I agree that all you listed seems like an awful lot to carry, but it’s always better to be safe than sorry, isn’t it?
There are patches that don’t require vulcanizer—the infamous peel-and-stick. One brand, the Park Tool GP-2, has some genuinely enthusiastic endorsements, so we tested it. I applied Park Tool patches to four different tires, at three different psi levels (60, 90, and 120). Three of the four didn’t hold—two released within minutes. The fourth deflated overnight. I redid the test, but repeated a second time, they all leaked within a day.
A plastic garbage bag, or $2 disposable poncho:  Just in case of an unexpected downpour or getting caught out after dark.   More than once, I’ve gone for a ride on a beautiful day with a perfect forecast, not taken a jacket and had the weather go south on me.  Being able to cover up can mean the difference between just mild discomfort or a freezing cold, shivering ride home!
The thumb test is the fine art of pressing on the tube with your thumb to see if it’s inflated. During testing, I took a pressure reading out of curiosity, and to the touch, road tires feel fully inflated around 30 psi. How pathetically sad—30 psi!—when you’re probably trying to reach at least 60 psi on a mountain bike, 70 or more on a hybrid, and it only goes up from there.

Nice, helpful, friendly. Central location. Good bikes (I had two over two days.) Bikes had a water bottle cage (some places don't do that!) Provided helmets if needed (I didn't) and a one-sided pannier containing a decently strong but not super-heavy cable lock and a patch kit. Only quibbles: 1) The owner helping me with getting my bikes seemed to 'drop me' for too long when someone else walked in. He should have said to them, after asking what they were in for, that 'I'll be with you as soon as I'm finished with this person.' So I had to wait a little longer than desired, but not bad. 2) On my second day, the pannier bag fell off and got lost after I stopped for lunch, which required locking up the bike somewhere, so I took the bag into restaurant and re-attached it using the two velcro straps on the rear rack. I've used many panniers before, thought I did it correctly. But apparently I did not. So I paid $55 (cost) to replace everything in bag and bag. In hindsight, I wished I'd put a bungee around bag straps to really secure it to bike. PROBABLY MY FAULT. 3) Bring a second lock to lock quick-release front wheel, as decently strong but not super-heavy cable lock isn't long enough for both wheels and frame.I had a second cable lock.

Brakes come in a variety of designs depending on the age and style of your bike. Rim brakes offer more modulation but have less stopping power, so are more suited to riding on the roads. Disc brakes can provide huge amounts of stopping power so are best suited to off road riding, although ‘V’ rim brakes are also well suited to mountain biking. All brakes use a friction system to stop the movement of the wheel, so regular checks of the contact pads should form part of your regular cycle maintenance.