Tom, I know its pointless to argue, and I agree the core pieces of DNN mainly the modules are pretty bad at this moment. Don't think I have used a core module for anything, they are just overly lacking. But on the flip side I have spent 100 bucks that is it for a module package which allows me to do everything I could possibly ever dream about needing. The bonus I have experience over drupal/Joomla is DNN is asp.net / sql based. A key aspect for many developers / business. While drupal / joomla are apache/mysql based with the possiblity of datalayers allowing access to sql it still cuts out the newer technology aspects of asp.net. I run a hosting business and have tried both Drupal and Joomla both cause well good SQL servers are expensive to rent / buy. In the end I moved to DNN cause of its structure. I run 3 main installs and have around 80 portals(sites) running off those 3 installations. Not they are on different servers for resource reasons (naturally). With drupal / joomla I had to keep installing new instances to have completely different user experiences. With DNN I have built in control to sub-charge / wholesale my hosting directly in the portal, turn on or off my custom modules and charge monthly access fees for a page, all with the core components.
For the table based comment its the community not the developers that cause you to see table based skins. Not a single one of my skins has any table tags they are all pure CSS and was really easy to do no issues at all, and I have tons of custom containers / modules running that I didn't build. DNN's skin parser is just simple enough anyone with notepad who knows how to code about 10 lines of html can make a custom skin for DNN and it will work. This means people can use a free wysiwyg editor and create their own skin in minutes via tables. When it comes to CSS skins well most people dont have a mastery level of CSS so they run into issues. For any professional web-developer with CSS skills its nothing and simple to do.
DNN's biggest downfall to date is its attempt to be backwards compatiable. Drupal is not at all backwards compatiable and is one of the main reasons I checked it off my listing. Everytime they released a new version half my modules stopped completely working. I think the core team has realized the main faults of DNN and are working to resolve them with version 5.0 most of the key flaws / holdbacks (caused by being backwards compatiable) are being resolved in the next set of releases.
Overall Joomla and DNN are close in features and functions, but as I stated one uses php and mysql the other is asp.net and sql. In regards to speed, its all about the server setup for a DNN site, and for those less techincal there is a free module which manages the viewstate issues of DNN basically removing its core slow spot.
Its really just spliting hairs, cause both systems do the same exact thing just different ways, once you truely learn either other you can out do the other one ten fold.
For me I hate how php is structured its super sloppy and all over the place. (note you can write good quality php but most people dont have that skill level so you get a bunch of sloopy coding). Asp.net forces a unified structure and no matter what asp.net code I look at its almost textbook standard.
Is either bad no, they both accomplish the same thing, just preference.
Back to OP: you can wait but I suggest going ahead if its needed and getting a module via 3rd party and using it, till the full end version of DNN 5 is released. Noting it might be end of year or later.