I won't get all the details right, but these are the basics. About 6 months ago, BA changed its seating policy to significantly restrict the number of passengers eligible to select their seats in advance at the time of booking. If you're BAEC Gold (essentially the highest level), you can still select seats in advance for yourself. I'm not sure if it's been corrected yet, but there were, however, problems with selecting preferred seats if anyone in your party was less than Gold. You can also select seats in advance if you're BAEC Silver, Oneworld Sapphire or Oneworld Emerald (although you don't have access to certain highly desirable seats), have booked a First class ticket or have a full fare ticket in any other class. (Note that First is long-haul super-premium, not J/business class.) If you and your travelling companion are on separate PNRs (e.g. because one of you has a revenue ticket and one has a reward ticket), there are problems getting seats together (if your travelling partner lacks status, s/he might not even be able to select a seat at all at the time of booking) - this one might be in the process of getting fixed, but I think there are still glitches.

In other words, you could spend several thousand dollars (or more) for a slightly less than full fare J (Club World) ticket and not be permitted to select a seat until 24 hours in advance. (Since most J class seats are pretty good, this is not the world's greatest hardship, but not value for money in anyone's books.) I think the seating policy really bites with World Traveller Plus, where you pay a significant premium over Y (but still might be holding a less than full fare ticket) but could end up in the middle seat immediately in front of a toilet. Likewise, the policy is a pain if you've booked a reward ticket but don't have elite status.

People with disabilities (and members of their party) can also select seats in advance but "disability" has been interpreted extremely narrowly (i.e. basically, if you have to be carried up the stairs to the plane), with the risk that people with significant disabilities that don't meet the test being separated from their travelling companions. People travelling with small children can select seats in advance, but only about 3 days in advance. I'm not sure about Oneworld elites. Everyone else has to wait until 24 hours before boarding. If you check in on-line, people at check-in and fast bag drop desks can't change your seat later (so if you accidentally get stuck with a lousy seat, you're stuck with it forever).

I've done OK so far because I have BAEC Gold status and Mr Canuck has Silver status. However, if I lose elite status (and I probably will next year), I plan to stop flying BA as soon as I unload my miles.

There are several threads on the BA board discussing the application of the seating policy.

Combine that with the British Airport Authority's "one bag means one bag, not one bag + computer or one bag + purse" and a lot of business travellers have decided to spend their money with someone other than BA.