I would be interested why this is what did it for you? I work at Rdio, which is a competitor that has been HTML/JS/CSS with a flash player since the beginning. What about this makes you like Grooveshark more?
I actually have an Rdio account, because I have a curious bias towards web sites with sexier names/subconsciously believe I am judged by the names of web sites I use. When I joined I had the impression that Rdio would only let me listen to unlimited songs if I paid you money. Now I see that's not the case, so I'll give your service a go.
First thing I don't like is that you don't give me a big search bar. I'm not paying you money. I don't want to log in. I even resent Grooveshark's making me log in to share songs, that's how lazy I am. (I do have a Grooveshark account, which I rarely use.) So the fact that your front page looks more like a product than like an easy service, that scares me away.
I think the two most common-looking site designs are: search bars and product pages. Product pages are universally ugly and I run away from them actively. Search bars I will type in something and see if the engine meets my whims. So Grooveshark's front page sells more.
Rdio fails the Cardiacs test: http://www.rdio.com/#/search/cardiacs/ returns nothing, whereas Grooveshark gives me almost their full catalogue. Cardiacs is my general test of band obscurity: If you have them you're bound to have most of what I want, and if you don't then I trust you less. (You both fail my Victoria Pipe Police Band test, which is pretty sad, because I request my fervent demand for pipe music be sated. >:( )
When I go to a band's page, I get something that looks like informational bullshit. Which is nice — I use Last.fm for my band-related nonsense — but when I want a music player I want small entries that show me as much music as possible. If I search a band I want three albums on my screen simultaneously. Grooveshark gives me a LOT of music, and fast.
But here's the clincher:
When I go to play music, you ask me to sign in or register. BAM! I'm looking for a new site. I don't like giving out my email address. I don't like picking new secure passwords. I will only do it if I think you're worth the effort.
Right now I use three sites to listen to music. I use Grooveshark, or I use Bandcamp, or I use TheSixtyOne. Occasionally I use Pandora but that's WAY rare. Grooveshark's for general music hunting; Bandcamp is for searching for and buying music, and listening to musicians I know use it (like Sufjan Stevens); TheSixtyOne is when I want to explore random music. Each one is designed specifically to mimic my usage pattern. They look the way I want them to look.
Rdio looks like a college design project. (I'm not saying it looks bad. I go to a design college.) While it's pretty, it's not very functional — you have a lot of design choices that go against the way I'm trying to use the site. Kind of like when search engines make gangly content-crammed designs that stop me from actually finding what I'm searching for. I'm going to stick to the stripped-down tool that does what I want when I want it.
If you want to compete with Grooveshark, go slimmer, more efficient, and sexier. Make it easier for me to find the music I want than Grooveshark, easier to share, easier to — anything. If you don't, I have no reason to even contemplate switching. The good news is that I'm not loyal to Grooveshark beyond my affection for its design; because I don't have an account or use more features, I'm not entrenched. If you do something convincing I'd consider moving over.
Feel free to discuss this more with me, either here or in email. I love discussing design philosophy.
