What does it mean?

Just installed and now I am being denied great gaming!

As a Diablo franchise gamer for the last 10 years or so (since Diablo 1 came out), I can safely assure you I have no idea.

Is it legitimate (did you download it, or buy it)?
If it was (illegally) downloaded, it may be missing important files.

It may be incompatible with another program you're running?

It may think you're going to be another person who chooses the freakin' Barbarian character?
*fapfapfap* at Diablo II.
That doesn't sound too familiar. Could you take a screenshot of the message perhaps?
maybe you're hacking. or there's a firewall type thing blocking it. all i know is, d2 is the best game ever made. cannot wait for d3!
Jackson RR5 ivory w/ EMG 81/85
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Fender Starcaster Sunburst
Mesa/Boogie DC-3
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Ibanez TS-9
Morley Bad Horsie 2
Boss CE-5

ISP Decimator
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Korg Pitchblack
I will post a screen shot, but it's a legit copy, had it since 2002.

Maybe I have used my CD key one to many times?

But that doesn't make much sence to me.