OK, so I am sure this is easy to answer, but what are those messages I get which say "John Spartan's sticky launcher has achieved the rank of tactical nuclear missile" or similar?

Also how do I get my weapons ranking?

Only weapons that come from opened crates (called "Strange" weapons) can rank up. The rank is increased by killing players with the weapon.

You get 'strange' weapons from certain series of crates. Only strange weapons have ranks, as you get more kills with the weapon, it ranks up.

It seems completely pointless to me and the notifications that pop up are very annoying. I don't know why Valve keeps adding in this kind of crap.

I think it's pretty good. Makes even the duplicates from crates useful and I'd love to know how many kills I've racked up with MY Eternal Reward.

But I don't want to know.

It is quite annoying, but doesn't happen regularly enough to really make me rage. I would however like to see the ranking notifications happen between rounds.

Oh, and how much are crate keys then?

You can disable them from advanced options, it also turns off the trade request notifactions.

But I like trade request notifications

Exactly. It's another relatively cool feature that Valve just haven't thought through properly.