What to look for when buying muffin tins?
In general, the darker the color of the metal of the muffin tin the more it will brown its contents.  You also want to look for a thick, durable metal as this will aid in even cooking.  Finally, look for a tin with large, wide handles; the last thing you want to be doing is sticking your thumbs into a muffin when trying to insert/remove the tin from the oven!  Most muffin tins these days are non-stick, so it may be hard to find one that is not.  With that said, non-stick technology has advanced a bit since the Teflon-only days.  For example, many modern non-stick pans/tins instead use other metals and ceramics that are not known to be harmful.  Furthermore, some modern muffin tins are marketed as "metal-safe", meaning that one can use a metal utensil on them (e.g., to extract the muffins from the tin) without damaging the non-stick coating.  Finally, muffin tins will likely never be heated to the temperatures at which the "dangerous" non-stick coatings release harmful gasses.