I have a massive list of things, some of which include...
- The metric system. Almost certainly the most irrational. Especially since it is easiest to use.

- People asking me "Would you like to do XYZ?" when they really mean "do XYZ now" No I fucking would NOT like to do it; I'm only doing it because you damn well told me to!

- Text-speak. My mum has started using it and I struggle to decipher about half of her texts.

- American spellings. What was wrong with the originals?

- Using "percent" instead of "per cent". Again, a totally irrational one.

- The number of times I have to use the back-space key because my computer keys are so small.

Edit; add these.

- Having to edit my posts because my useless lard-lump of a brain won't re-call all the relevant information when it is needed.

- People who write "i" instead of I. There are two shift keys on most key-boards; please use them.

- That cat from two doors down called "Daisy" who comes in, eats all the food left for our own cats, hisses at them, sleeps on whichever bed it wants then leaves.

- Companies like Apple, Microsoft and Tesco who claim to be American (or British); most of their manufacturing facilities are in China.

- iPhones, iPods, iPads, eReaders, (in fact any of these things with the capital letter in the wrong place).

Finally...

Companies who have Facebook or Twitter addresses in their adverts and assume that every person on the planet has either. As if I'm going to join the Royal Marines purely because I saw them on Facebook.
"Status update: Just blown a man's head off LOL"
( Awk Industries is actually a helic- NO, STAY BACK SHARK!, Sun 3 Apr 2011, 21:50, 3 replies)

American spellings
Actually, the American spellings are the originals. They really should drop those old archaic spellings and use the modern ones we use. That and use international standard paper sizes (e.g. A4), dates in the format dd/mm/yyyy, and the metric system!
(An eejit passing by in a purple satin skirt with sparkly bits, Sun 3 Apr 2011, 22:42, closed)

Original spellings?
Words like "honour" come from Latin; yes the Americans do have the originals there. But words like "centre" come directly from French so we do have some of the original spellings. I think. As for the date format, that annoys the hell out of me. I now make a point of saying "It's the third of April." rather than "It's April the third." April the third sounds like it could be either a date or the name of a monarch.
( Awk Industries is actually a helic- NO, STAY BACK SHARK!, Sun 3 Apr 2011, 23:00, closed)

Tut! You can't believe anything you see on TV any more...
I saw a programme ages ago where they explained the differences as being due to the Americans using an obsolete pre-War of Independence dictionary. Oh well.

The dates thing really gets me when it's on stuff like backup software. The number of times I have tried to recover something, put the date in as dd/mm/yyyy and its come back with "nothing backed up for that date". Usually accompanied by a horrible sinking feeling as my brain goes "shit fuck arse bollocks! the backups are screwed!" then followed by the realisation that American dates have struck again and that everything is fine.
(An eejit passing by in a purple satin skirt with sparkly bits, Sun 3 Apr 2011, 23:15, closed)

I'm fairly sure it was Collins
Who tried to create a version of English with easier spellings for a population of immigrants mainly from countries that don't speak English.

I may have dreamt this.
( sandettie light vessel automatic New Twitter - @bollocksreally, Sun 3 Apr 2011, 23:17, closed)

Ah, hang on, I forgot these too...
- Words like "realise" being spelled as "realize", despite the fact that both spellings are acceptable in Britain.

- Having to go to bed, yet sleep is essential for human survival.

- Any musical "artist" who uses that "Auto-tune" shite.

- People who describe something as "an [h]istoric occasion" as opposed to "a historic occasion".
( Awk Industries is actually a helic- NO, STAY BACK SHARK!, Mon 4 Apr 2011, 0:08, closed)

American spellings
Do you mean things like that old favourite of narrow minded whiners everywhere; "aluminum/aluminium"? The American version is the original, everyone else uses the version which has a changed ending to match most of the other metals, e.g. uranium, sodium, etc.
( SteamedCleaner, Mon 4 Apr 2011, 5:11, closed)