I implemented wp's idea about merging the "Install" and "Update" buttons (r. 57328), now the package installation is less confusing. Thanks again wp. One limitation still exists though, namely the packages must be installed from the central repository first. Technically OPM can install directly from external source, but I prefer to keep this separation mainly because:
1. When installed from the central repository every dependency is resolved correctly. I test newly added packages both on windows and linux, with the latest stable version of lazarus. The packages are usually stable releases, except a few with slow release cycle like: synapse, indy, etc...
2. The user will be able to distinguish between a safe source(repository) and a less safe external link. Maybe I'm a little bit paranoid, but I really don't want to have any legal issues(although highly unlikely), with packages installed from somewhere else.
PS: I will not implement package install from SVN or GIT(at least not in the near future), because in my opinion it will cause more issues and confusion. Thanks everyone for the suggestions.