We have 60 Macs (Mac Pros, MacBook Pros, and MacBook Airs) purchased from 2011-2015. We see all the problems mrmondo mentioned except the iTunes one (we don't use iTunes).
The true deal breakers for us, though, is the terrible Wi-Fi performance. The Macs get spurious Wi-Fi failures; all Windows, Android, and iOS devices work fine on all our wireless networks.
As a result, about 45 of our Macs now run Windows 7 or 8. That means the users get a worse UI and not nearly so much functionality built in -- with the tradeoff being that the machines run predictably and reliably.
It seems everybody now has their "back in my day..." OS X version when things were stable; for me 10.7 was the last good version. All our problems started with 10.8, and things have been going downhill since then.
You can suspect that but you'd be wrong. My early 2013 15" rM P experiences most of the issues the above poster describes. It also experiences graphics subsystem freezes even when the only app I have open is iTerm, requiring a power button reboot. Windows Server service can often be seen chewing 30% or more CPU, Wifi often needs to be turned off and on, while in range of my router (latest gen/firmware Express and Extreme). Every day I think I should go back to 10.9, but I'm worried for the day, if not already, that Apple refuses to back port security fixes (which may even have happened, recently). Especially a worry for me since I am involved in healthcare in my laptop.
Firstly I would try creating a new account and seeing if the issues persist.
Also one trick is running "sudo fs_usage". Often misbehaved processes that are hammering the CPU are also doing the disk as well e.g. outputting lots of Console logs.
Most of those issues sound OS related. The author is presumably technologically mature enough to check activity monitor - indeed, they probably have to live in it to kill their errant processes.
Activity monitor shows you Disk, Network, and CPU activity, and is a good first place to see if anything is going awry when your system is acting up.
I'm interested in how you might use fs_usage? It actually is a new utility to me, and when I turn it with fs_usage -e iTerm - I get a lot of activity, on a basically idle system. (At least one that Activity Monitor/ioStat shows as idle).
Are you 10?
Seriously, when did you start using Operating Systems if this is the buggiest, most unreliable OS you've ever used.