This release is primarily a bug fix and performance / memory usage release. Over 70 fixes and enhancements have been incorporated since 1.5.0, you can see the list through this bugzilla query.
AspectJ 1.5.1 has significantly lower memory usage than AspectJ 1.5.0, especially when building inside of AJDT. The improvements are also noticeable when batch building with Ant, or when load-time weaving. Batch building a project with over 1000 source files with both AspectJ 1.5.1 and AspectJ 1.5.0 shows a 16% reduction in peak memory usage, and an 8% reduction in heap consumed by temporary objects. The AspectJ 1.5.1 compiler integrated into AJDT gives memory savings of at least 50%. The greater savings in the AJDT environment are due to improvements in areas such as structure model generation that are not exercised
For information on bug fixes in AspectJ 5 v1.5.1, see the changes document.