I always like the week between Christmas and New Years to focus on cleaning up our instance from the past year so we start fresh. These are reviewed as needed throughout the year but this is the 'Big List'. My list is building but here's a snapshot of what I mean.
Reports not used
Reports shared with
Reports 'Run As'
Team Specific Reports
100% but not complete
Data Points - Growth
Data Points - Institutional / Retail Sales
Last Logged in users
Created 'Shared With' View to see projects
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e.g. Workfront considers Portfolios an object, so what can I say about this from a maintenance perspective?
Are the portfolios still organized according to your governance charter (however you chose to set them up)?
Are they still being used?
If they aren't, are they set inactive or deleted?
Are they up to date? (owned by legitimate users)
... and so on and so forth: programs, templates, projects, tasks, issues, documents, document or proof approvals, approval processes, milestone paths, reports, dashboards, users, groups, teams, roles, layout templates -- the list goes on -- what is it you can say about each of these objects from a maintenance perspective?
The great thing about Doug's list is it's already pretty organized in order of "most bang for buck"... but that's just my opinion.