Power connections
As durable as mains cables are, sometimes they do get damaged, and if you're lucky, the best you can salvage them is by cutting the damaged portion off and installing a rewireable plug. This leads to a cord of a random length, always shorter than it originally was. And even if it remains intact, it may be the wrong length for the setup or (if you imported it) have the wrong plug for your region (for the purpose of this statement, it is given that the voltage and frequency match). If it's too short, you have to use an extension cord, complete with often-bulky socket. If it's too long, you have to take up the extra length. Neither is neat. And, of course, with the wrong plug...
This of course is what IEC60320 connectors are for, but a lot of manufacturers refuse to use them for some reason that escapes me. Some of them even design their own connectors (which, again, escapes me).
Many external PSUs have the plug as part of the unit itself. This is a major problem as many of them are bulky (or with switchers, simply the wrong shape), obstructing adjacent sockets. Their other problem is with the common barrel-type connectors, which are not keyed for voltage (19V notebook charger into an external drive? Hope you have a backup!), the polarity is the manufacturer's choice and they are used for both AC and DC.
Colour temperature
Despite being critical, it is all-too-often overlooked (pun unintended), which results in light that's either too orange or too blue. Additionally, although 2700K (often referred to as "warm white"; meant to approximate the light from incandescent bulbs) is often recommended, I won't choose it as it's way too orange and clashes hard with sunlight. I think around 4000K is ideal. No major deviation in either direction is acceptable.
Colour saturation
The video display marketroids just had to decide that "more colour = better". Presumably the trick is to fool people into thinking that they're capable of more vivid colour, but that's not it, as any such display can produce bright colours at will. The effect is that the colours are over-saturated, which is unrealistic. On a side note, the "brightness" control on many modern television sets just isn't - turning it down washes dark colours into the black, and turning it up blends the image into white. And I can't stand high brightness (which is the case for most modern displays). It wastes power for no benefit and is harder on the eyes than lower (within reason) levels.
You can post more examples, and comment on those already mentioned.
