youJoomla! - Not too complex, easy to add, although they are more integrated with 1.0 than 1.5 right now. (We assume that will eventually change.) Bold colors, large fonts, diverse look and feels. I decided to join them because of a couple of the looks, and in the end haven't found a use for any that I've downloaded yet :)
Negative: Their Contact Us form is for billing inquiries only. Pretty much no way to contact them -- unless of course it's about billing.
Rocket Theme - Templates run more into clubs, associations, younger generation. They have a few templates that will appeal to an older generation of user, but most of them would have to be highly customized in order to do that. There are several good business templates, but none that I would use out of the box. That said they are HIGHLY customizable, and have a ton of options to work with.
TemplatePlazza - Younger generation, I would use these more for teen sites, or schools, anything that would be for a pre-teen / teen / early 20s crowd. Most of the templates are more in line with a cartoon or caricature feel. Good functionality, but very big iconology with lots of mouseovers.
yooTheme - Artistic, good functionality, more interested in how they combine their extensions with the templates myself. I find that their templates are very specific, I can look at them and immediately know if it would work for my client. This is good on one hand, but customization for a "harder" client is difficult.
Joomla Junkie - Very nice diversity, it looks like they work with a lot of different designers, so the templates all have different looks to them. One of the problems you usually run into with a template is you can tell immediately who it came from because of the styles, I find that Joomla Junkie has a lot of look and feels to choose from. They are simple to use, simple to update. There are not a lot of templates yet, but they are always adding to the selection.
Joomla Bamboo - These templates are really very nice, a little more difficult to update than some, but once you get the hang of it, you can really make changes to each one that completely change the look and feel but keep the layout. This is another one though that is pretty specific per template. It's a blog, it's a portfolio, it's a business site. I wouldn't say that all of their templates are multi use.
JoomlArt - This is a very artistic type of template. Lots of diversity, easy to update and a lot of templates to boot. They are changing how they do things, and instead of giving 2 templates a month now, they are only going to be providing one per month, but there are so many of them already, and they always keep them up to date, that it's not a big deal I don't think. This is one of my favorites to start with.
JoomlaPraise - this one is relatively new, but I like how they are doing things - slightly reminiscent of yooTheme in the fact that they are developing some nice extensions as well, very bold colors and ideas, I will know more once there are more templates available.
joomla-templates - I have had people tell me that they like these templates, but I haven't found any use for them. I think they are easy to find, and they are free, so they attract a lot of attention.
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