My Vista install turned 5 months old this week. I think I have enough perspective now that the initial gripes have evaporated, leaving only the true issues to bug me.
What frustrates me about it?
Aero Glass is agile like a limping cow. Nothing is ever instantaneous in the UI, reminds me of Mac OS X. Even simple alt-tab is much slower than it used/needs to be.
The thread scheduler isn't very fair. If there's a single thread that wants 100% of the CPU, it typically gets it. Meaning every other app in the system barely gets to update its UI. Multitasking falls on its face... I have to play the Task Manager Priority game.
The CPU is always busy doing something. God knows what. I'm playing music in iTunes and the CPU is at ~40%. XP needs ~5% to do the same thing.
The sum of the CPU times in the Task Manager doesn't always add up to 100%. Probably because services are hidden. I want to see which bugger is taking my cycles away and I'm going to clobber it.
The system randomly fails to get out of sleep mode, ~10% of the time. It's gotten much better since I re-enabled the pagefile.
Almost all performance issues, then a bit of [un]reliability.
Let me tell you what I have no issues with: everything else. I have no issues with the security features, I leave them all turned on. I have no issues with the UI, overall I like it. Vista is the best Windows release ever at networking; I used to rage against XP's incompenent treatment of wireless networks, and now I haven't had so much as a single stutter in 5 months of Vista use.
I think I like Vista very much, I just wish it wasn't so slow. I dare not subject Vista to even half of the workload XP handled for me every day in the last few years (3~4 Visual Studios, a dozen Internet Explorers, a dozen Windows Explorers, Outlook, Powerpoint, my own software, iTunes, ... all at once). Maybe one of those 8-core monsters will be able to run my workload.
I suppose none of this will matter in ~2 years, when giving a whole CPU core to the OS will be a no-brainer. It's just right now, I only have the one core.