Thus whenever I've reviewed a game, I've looked very carefully at the design of the components, and in particular at their usability. Below I've listed about 50 of these lessons learned. They're not necessarily the 50 most important; I'm sure I missed the third biggest thing that any component designer should know. Instead they're just the first 50 or so items that came to my mind first and/or the ones recently mentioned by other gamers, by my wife, or by myself in my last 9 months or so of reviews.
Feel free to add on your own in the comments section, and maybe next year I'll update this article with a new and improved version!
Box DesignDo use cardboard for your boxes, not cardstock, unless you're making a relatively small card game.
Do consider the very shallow boxes now being used for games like Hansa and China. You get plenty of space for a board, and then a more appropriate amount of space for everything else.
Don't use a vastly oversized box and especially don't try and disguise the vast oversizedness by putting in a plastic tray that takes up 90% of the box.
Don't put out every one of your games with a different sized box and different branding.
Also see The Problem with Game Boxes and The Collector Bug for more of this rant.
Cover ArtworkDo have beautiful & evocative artwork that gives a feel for the theming you're trying to convey.
Don't print artwork that might be offensive to any large constinuency that you want to sell too. Cover nudity doesn't go over well in American, nor do slaves in chains, and especially not a combination of both; swastikas go over particularly poorly in Germany where the postwar criminal code actually makes it illegal to publicly show them, except for scholarly purposes.
Tray DesignDo include finger holes for any set of tiles, hexes, or cards that you provide a slot for, so that players don't have to dump your box to get the pieces out.
Do figure out what happens to the pieces in your tray if your box gets set on its side, particularly if your game is bookshelf sized.
Don't inset tray dividers notably below the level where the map or rulebook will set, else you're just asking for pieces to go all over.
Game BoardsDo fill the unused parts of your board with rules that players might not easily remember, like anything you'd put on a player aid sheet (see below), especially including victory point tables, action point tables, and combat tables.
Do try and minimize the number of folds in your board. Four panels is expected, two is really nice, and six is excessive unless you're producing a really big board.
Do use your board to pump up your theming. Named spaces and unnamed spaces have no functional difference, but the former can make a huge difference in how people vizualise & enjoy your game.
Don't go so artsy with your board that it becomes hard to use.
Don't make grossly large boards that are mostly or largely unnecessary. Most players will prefer a smaller board (and thus a smaller game) over a large one and will prefer no board at all, to allow for a card-sized game, if that board really wasn't needed.
Scoring TracksDo include a scoring track for your game, even if players just need to add up a couple of different things at the end.
Do try your best to make sure your scoring track will accomodate the highest score you expect players to earn in 90-95% of games.
Don't create a scoring track that players will have to wrap around 3 or 4 times.
Don't have your scoring track wrap at anything but a power of 10.
Don't even think about creating a scoring track where players mark their 10s digit with one marker and their 1s digit with another. It's unclear, easy to mess up, and doesn't do a good job of intuitively showing the rankings in a game.
Wooden BitsDo make sure that your wooden bits are the right size for the spaces they go into. Don't make them too big, even by a little bit!
Do try and use the most appropriately themed wooden bits you can, especially if your game has multiple wooden bits that get put in the same places. The more inappropriate wooden bits your game has, the most opportunity there is to mix them up.
Don't use those darned wooden cylinders which hit the table, roll, then bounce straight onto the floor, where they're eaten by the family dog.
Plastic BitsDo make sure that your plastic bits are the right size for the spaces they go into. Don't make them too big, even by a little bit!
Do cut your plastic pieces off the sprues before they get to your customers. No one likes to spend 2-4 hours doing so on their own.
Don't make plastic molds which such ridiculously small bases that the resultant pieces just fall over.
CardsDo consider the readability of your cards not just in the hand, but also from across a table if your cards will ever permanently be played to the table or drafted from the table. Icons can help a lot here.
Do make sure that your most vital card information is visible when your cards are fanned. This means that you shouldn't put the info that players will use to select the card in the middle of the card or most of the way down an edge.
Do duplicate that vital information on two or four corners.
Do carefully consider the layout of your cards if they have any complexity. Similar layouts for different types of cards can highlight similar or identical information by putting them in the same places. Different layouts for different types of cards without any similarity of information can make that differentiation that much clearer.
Do think about ways that your cards can be stacked on the table to preserve information yet save space if there are going to be a lot of cards on the table.
Do be very careful about your card backs. Don't use the same card back for different types of cards unless there's a good reason for doing so (usually so that players can hide which types of cards they have in their hand) and really don't change your card back in a supplement or expansion, even to a slightly different hue.
Don't use a thinner than normal cardstock, and especially don't use a partially see-through cardstock.
Don't think that icons do the whole job. Text and icons are good as backups of each other. Unless your icon designer is a genius anything else will impact the usability of your game, and that's not worth the internationalization benefit.
Don't, for Knizia's sake, even think of printing a card with no explanation of what it does, assuming that players will memorize the list of card abilities from the rulebook.
TilesDo invest in good die-cutting, especially if you're producing a tile-based game where it's important that tiles sits flush up against each other.
Player AidsDo include player aids in your game, particularly if different units have different powers, if players have action points they can spend on different things, or generally if you have more than one of two things that players have to remember. Look at The Settlers of Catan for the best way to do this sort of thing or Memoir '44 as a good example of player aids for different terrains or Java, Tikal, Mexica, or Torres as good examples of how to summarize action point usage. You can also be clever and sometimes include this stuff on the map, as noted above.
Do figure out how to quickly summarize your player aid information using icons and pictures.
Don't produce player aids that solely have huge blocks of text summarizing what to do, because they generally won't get read.
Color UsageDo seriously consider using the most common four colors--red, green, yellow, and blue--as four of your core colors for a game, because players will have an easier time remembering what color they are if they often play the same color.
Do include some permanent reminder of what color each player is, be it a player aid, a special token, or just the expectation that each player will always have some pieces of his color in front of him.
Don't use multiple colors in the red-pink-orange spectrum.
Don't use multiple colors in the blue-purple spectrum.
Don't use the same colors as a player color and for some other purpose (e.g., a goods color, a track color, etc.) in your game, or you'll confuse players.
Also see The Problem with Colors.
RulebooksDo include pictures and examples, as both make rules easier to understand.
Do include summaries and do pull out important rules into sidebars.
Do consider an index, if it might be at all appropriate.
Do include a glossary for any cards, tiles, or other items in your game which might have special powers. This is a great place to address any weird questions about those individual items and about interactions between them.
Do look at how Alea does their rulebooks and mimic that, as they make the best in the business.
Don't include new rules in examples.
Don't have a bolded, named section, then include some of the rules related to that name elsewhere--unless you have a reference in the text.