One thing that's always been a quite controversial topic in SL is the creation and usage of megaprims. SL itself imposes a limitation on the physical size of any prim, which is 10x10x10 m at maximum.
Due to programming gaps and errors though there've been people around in the past, which created found ways to overcome that limitation and created bigger prims than that. Since you are unable to create those prims by normal means, those have been sold or just been given away in big packages for free and still are. In the times of Havok I being the physics engine running all over the grid one of the main argument against the usage of megaprims was that it creates unnecessary lags.
Anyway, many people just didn't believed that or cared at all and used them anyways. With Havok IV in use now, those issues about megaprims causing lag are clearly outdated say Linden Lab itself. Also already around two years ago Andrew Linden hinted they might be allowing the free creation of those prims in the future.
Considering the fact, that megaprims are wide spread, here to stay and freely creatable in Opensims, the free creation of megaprims in SL is one thing I'd really like to see happen in 2010.