Five or six to nine speeds in the rear is a definite difference, and the advantage is on the nine. I set mine up with a corncob in the center, three climbers on the big side, one killer gear (rarely used) on the small, based around the middle gear being what I'd normally use with large chainwheel on the flats. Yeah, I shift a lot more, but the difference in performance is worth it - if that's how I'm riding that day.

Five to six is nice, as it gives you two gears in the center, rather than one, for those stretches with some minor climbs on them.

Six to seven? Meh. Big advantage there is that you get to start using brifters.

Six to eight? Not much over seven. It takes a three cog difference to really notice any real performance differences. I've yet to use 10 or 11 speed cassettes. Too expensive.

Yeah, downtube shifters are cheaper and simpler, but when you're screaming down a hill at 30mph and setting yourself up for an immediate climb as soon as you reach the bottom (common in my area), there's no way in hell I'm taking my hands off the bars to shift. I've already had one broken wrist.

I notice that when I'm out with my usual Sunday morning ride in Ashland (Poguemahone and two girls not on the forum), and I know we're going to be playing hard-ass boy-racers that day, I take the modern bike with brifters. Period. They're more efficient, so what if they're more expensive and not repairable? Yes, I shift way more often. It's nice to be able to fine tune the gearing to match the terrain and your cadence, even if it is every 500 meters.

My liking for cassettes over freewheels is in the simplicity to rolling your own gearing setups. I've only ever taken one or two freewheels apart in my life to swap cogs. It's more trouble than it's worth. With cassettes, you get the ability to swap easily. Of course, the technological improvement is completely subverted by marketing. In a perfect world, a bike shop would carry a board with individual cogs and spacers. This is obviously not a perfect world.

Single vs. dual post calipers? That's the closest in the obsolete stakes for me. Singles still work decently well. However, if that chick texting while driving cuts a fast right turn in front of me, please God, let me be riding dual post calipers.