Nice article Steve ! The concept is very clearly explained in the right order.
I think if you can also add the "Disclaimer" which is a must I feel then the article shall be complete in all the aspect. Anyway, that's just a suggestion.
"Indexes aren't free and has associated cost in terms of maintenance, storage, IO, Replication etc." and "One should think mindfully before creating an index". First preference to be given for making full use of the available indexes. What I've seen, adding new index become the only solution for the DBA's and the Developers when they encounter performance issue.
Many a times no one really cares how many unused indexes they already have? and what will happen to this new covering index after they will change the filter/join/sort predicate or add any new column in SELECT. Although, you have covered few things as part of "Real life tips" which is really good.