Below are some myths and misconceptions about Joomla! you often read while browsing forums.
Joomla! is only for small sites
There are a lot of large Joomla! sites out there. In fact, some of the customers we do consultancy for, are building Joomla! sites for some well known multinational brands. Often these sites are customized beyond recognition, so there's no way of telling from the outside which software was used. There's also a good reason for keeping it that way: the less a potential hacker knows about your system, the better. So for critical sites, the developers go out of their way to hide the underlying technologies from their visitors. Finally, these developers are usually to busy to hang around in the forums.
Legacy mode is bad for your site
The legacy plugin is just a thin layer. It fools legacy extensions into thinking that they are running on Joomla! 1.0.x. Apart from that, it doesn't affect you system or other extensions, and hardly decreases memory use or performance at all. Some third party extension developers however use the fact that their extensions are 1.5 native as a marketing feature, creating the image that legacy mode is bad. This in turn has led some 3PD's to make fake-native extensions: these are in fact 1.0 extensions that don't require legacy mode, but include a copy of the legacy library in their own packages.
That being said, there are definitely benefits to using native extensions. But I'll use a mature, performant and secure legacy extension over a shabby native one any day of the week.
Joomla! has insufficient SEO
A site's SEO is what you make it, and no tool will do it for you. Search engine optimization is not black magic. Google and others try to find pages like a human would, so they also try to look at a site the way a human does. When you look at web page quickly, you will notice things like page titles, the URL, headings, ... When these contains words that describe the text, you'll already have a good idea of what it's about, without actually reading it. So well structured content will give you better page ranks than any tool ever will.
Again there's marketing involved: companies trying to sell you tools or services that will magically boost your ranking. For instance, the SEF URL's that Joomla! generates are more than adequate, as long as you pick your aliases sensibly (though admitted, they are less pretty than the ones some other tools generate).
Joomla! is for end users, not developers
When you're a web developer building a site for a customer, you'll be working on the project for a couple of months max. After that, end users will be maintaining the content for years to come. So a CMS's user friendliness should be your first concern. Just think of all the phone calls you'll get when your customer has trouble working with the system.
It's a history thing: Joomla 1.0 had a lot of messy code and often required hacks to get something done. And Joomla! has always had an active end user community, whereas projects like Drupal have attracted more developers early on, which has helped shape the whole notion of Drupal being more developer friendly.
In Joomla! 1.5 however, a lot -- everything! -- has changed for developers. Anyone who claims different clearly hasn't looked under the hood, ar at least not long enough to realize it's full potential. The new framework is very powerful and flexible, and allows you to build proper object oriented applications.