For one, the interface is a web site that isn't well-adapted to a mobile screen, so getting to the store is cumbersome in itself.
Secondly, downloading an app after buying it is very slow and frustrating. It involves adding a special APT repository with a unique key, and APT on the device is unusably slow.
Third, actually publishing on the store is, as far as I can tell, practically impossible unless you have a company and the name of that company is Rovio.
Third, they have no decent categories, no search functionality worth mentioning, very little information about each app... the list goes on.
As a final nail in the coffin, the prices are much higher than in the Android or iOS stores.
This is all from when I tried the store, needless to say I haven't felt any desire to relive the experience again so things may have changed. I really doubt they've fixed it though.
I don't know if you've used the store on a symbian device, the experience may be better there, but my experience was indeed terrible on the N900.
I really don't see Nokia turning around and putting out a fantastic mobile OS with a well-integrated experience that grabs customers, just like I have a hard time seeing Microsoft managing it. Although, Microsoft do have Xbox Live they could leverage.