- Tables are usually more bytes of markup. (Longer to download, and more bytes of traffic for the host.)
- Tables usually prevent incremental rendering. (Takes longer for the user to see anything on the page.)
- Tables may require you to chop single, logical images into multiple ones. (This makes redesigns total hell, and also increases page load time [more http requests and more total bytes].)
- Tables break text copying on some browsers. (That's annoying to the user.)
- Tables prevent certain layouts from working within them (like height:100% for child elements of
<td>). (They limit what you can actually do in terms of layout.)
- Once you know CSS, table-based layouts usually take more time to implement. (A little effort up-front learning CSS pays off heavily in the end.)
- Tables are semantically incorrect markup for layout. (They describe the presentation, not the content.)
- Tables make life hell for those using screen readers. (Not only do you get the other benefits of CSS, you're also helping out the blind/partially-sighted. This is a Good Thing.)
- Tables lock you into the current design and make redesigns MUCH harder than semantic HTML+CSS. (Have you seen CSS Zen Garden?)
Tables can be Good
Tables are 100% acceptable, appropriate, and correct for use with tabular data. For example, tables are the correct markup for the content of Filterlicious.
If you can't decide if your data is appropriate for a table, see "Tables are for Tabular Data. Period".