The problem with Drupal is that there is zero compatibility from release to release. Drupal 6.0 was released to the wild on Feb 13, 2008 and it took over four months for the state of Drupal to reach a point where upgrading to Drupal 6 was a reasonable option. The delay? Why, forward porting the important addon modules. Why? Because there is no compatibility and all the modules are broken. Further some highly critical modules, Views and CCK, are only just now getting well enough developed to think about using them.
Concurrent with the Drupal 6 release the Views team had this brilliant idea to drastically improve the Views module. They've done a great job and I'm pretty happy with it from a usability perspective. BUT IT IS FOUR MONTHS AFTER THE RELEASE AND THE VIEWS TEAM TIED THEIR NEW VIEWS WORK TO DRUPAL 6. Because Views2 would be the only version running on Drupal 6, this meant delaying adoption of Drupal 6 until Views 2 was ready. The other deadly embrace here is that the Content Construction Kit (CCK) is closely closely closely tied to Views and the Drupal 6 port of CCK had to wait for Views 2 to be finished.
As of this writing both Views 2 and CCK are in beta .. hardly gives me confidence but (knock on formica) they seem to be working on this site.
Another serious stumble is the migration of Amazon item support. I have a couple sites with hundreds of Amazon products listed on the site. The old Amazon items module is being abandoned in favor of a completely new one. This means the hundreds of existing Amazon product listings I have on the old site ... they'll have to be converted, somehow, someway, to the new module. Uh... Ah... As of this writing the new Amazon module is in Beta as well, due in no small part to the new module being tied to CCK support. As of this writing the author of the new module is promising a migration route for users of the old Amazon module, but that migration route is not currently finished.
The new Views module is fabulous, sure, yeah, it's wonderful. Creating a view under the old module was close to a nightmare of confusing parameters. The new Views module includes a drastically different user interface which is immensely easier to navigate, understand and configure. I understand it has immensely more flexibility under the hood as well. So in the end it may well be worth the wait.
The new CCK is also nicer and more flexible in use. It too may well have been worth the wait. I understand that some of the old CCK has been moved into Drupal Core and that in Drupal 7 even more will move into Core. I see on the Drupal site that there's already a CCK for Drupal 7 so maybe the upgrade to Drupal 7 won't be as bad.
One casualty seems to be the quicktags module. I'd used this on one site to provide a button bar that helps with writing BBCODE tags. However the quicktags author doesn't seem to want to continue maintaining it. However there is a new editor that appears to be promising, named markItUp. I've installed it on this site and it appears very promising.