1. Crashes. Of Course.
2. DP with heels. If I were in charge of programming in Second Life, this would be the top of the bug list.
3. Blue dialog boxes. Stack of ugly things in the corner interuptting everything. What would I do? I would have a window that has a "drawer" effect list of dialogs, small ones would have the yes no cancel options visible at a click or with check boxes to deal with several all at once, larger ones would, on mouse over, slide out, you could resize the space so that when you want very long dialog boxes would be visible. That or a wheel window would pop up with options or "more."
4. Pose balls. All right thinking avatars hate ball sprawl. What would I do? At the very least hide them. Better would be to make it so that more would position avatars above them, and appear as spots on the ground. I may do a demo of what I mean.
5. Lack of ability to create facial animations.
6. No joins on hands for our skeletons BVH files.
7. Slugly architecture. This is too big for a fix, but one major improvement would be to actually support huge prims. Another would be to have guides to better building published.
8. The Have/Have not economy. Too big for a single fix. But one large thing would be to make information easier to find. Traffic is a killer of information. It is meant to be. Get rid of it.
9. Ad farms. And all the things LL deliberately does to make them possible. Simple fix would be to zone mainland sims in advance by the minimum parcel that can be created and minimum width. Never create any more Governor Linden strips that create jagged edges whose sole use is for ad farm creation. That way, if people want to buy into ad farmy sims, and put up with the clutter, they can. Presumably at lower cost, since many will not want to be there. Right now everyone everywhere on mainland is always at the risk of slumification.
10. Its state religion.