Answers:
Q: What is it about the Apple platform that makes the web browsing experience so horrible?
A: I have no idea, but it's getting old. I would like to know as well. Downloading and page rendering on my machines is plenty fast on a DSL, but Flash is noticeably slower than my PC. However, A Shockwave 3D project I'm working on runs considerably better on my Mac (Dual 1 GIG vs. 800 PIII). I doubt Shockwave is DP aware. And don't even get me started on the whole window resizing thing. I've been complaining forever about that. As for Explorer crashing. I have never had IE crash once in OS X and I've been running OS X exclusively for a year. I do a fair amount of web development too. I still find IE the best browser. It simply works, like it or not. But that's a whole different argument.
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Issue #2: Launch of web services too much for Apple to handle?
I have .Mac, but I don't really use it. When I do, I find it slow in navigation, but reasonably fast at UL/DL once it gets going. I think .Mac at its core is a good quality service that helps to complete the Apple experience as an integrated service. .Mac is brilliantly connected to the OS (desktop mounting, nice). For me, .Mac has never caused trouble, so I think it is of value.
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3. Where is the future?
The future is steadily increasing performance on a constantly refined OS. I am very happy with my Mac today. It will only get better over time. Your point about the performance plateau is actually an argument vastly in Apple's favor. Apple has been very strategic, brilliantly so I might add, in positioning itself as the Digital Hub with a wonderful suite of very useful and usable apps. On the high end, they have again been incredibly smart in buying out quality companies and their software. As long as they can keep it all from becoming a hodge podge (like DVD Studio Pro is now), it will be a great move. This position as a smart, efficient tool helps soften the speed argument. I too am always lured by the speed of the dark side (I do a lot of video editing, 3D rendering, Photoshop, etc. so every bit helps). But I also know from experience that every time I move in that direction, I get burned. As for a "REAL reason to switch", I think Apple's digital hub and its simplicity and the fact that it "just works" is a damn good reason for some people to switch (I saw a couple in their seventies last month at the Apple store walking out with an iMac, their first computer ever). It's unfortunately, not much of a reason for a web developer to switch.
Final Note:
Most every anti-Apple argument is based on the speed thing. Yes, it bugs me too at times, but Apple is at the mercy of chip manufacturers that don't make the bulk of their money on said chips. It's kind of amazing that Apple is even able to be where they are given their market share. It really is. If Apple is ever able to close or come close to narrowing the speed gap, there will be nothing left to complain about except price, and I will gladly continue paying a premium for the best overall experience.