Hi,
I have marked this for Drupal 4.5 or older and 4.6, but this should probably hold for Drupal 4.7 as well, as I haven't noticed any discussions about this yet, to indicate that this is indeed being worked on. Both of these are serious usability issues that need attention, and I will do the grunt work if someone else can do the decision work and get some implementation ideas for it, as I am not a good enough coder to figure that out :-)
TabIndex is very important for blind people to use the Internet. I have noticed this to a great extend in my website created for a blind person at www.eagleeyes.co.za, which is a Drupal based site. It is, however, not functioning as required with Tab Indices as I would have to patch the whole Drupal distro to do this. My feeling is that EACH AND EVERY link should have a tabindex attribute added, even those generated by modules. The how of it all didn't exactly go through my head yet as this is a complicated process, especially to keep track of the indices allocated to each and every link.
The second important thing that has been discussed before is the use of title attributes for tooltips. There are some who say that not every link should have a title attribute, and those who dislike any tooltips, and then there are those who feel that a title attribute is needed with EVERY link. I am of the latter group, because a lot of screen readers make use of the title tags to read the text of a link. I also make use of this at www.eagleeyes.co.za (see the navigation bar). I believe it is imperative that we start a discussion about this, and work out a strategy for implementing such a wide change in Drupal.