Guccifer 2.0 Is Russian Because... Guccifer 2.0 Is Bullshit Because...
Meta data and error text on his documents contained Russian language. Russian language meta data appears to have been part of a deliberate process.
He used a VPN that didn't seem to be available publcly. ThreatConnect didn't realize they could just leave the dropdown at it's 'default' setting which would have given them the IP address they assumed to be private.
He was connected to DCLeaks and they've been attributed to an APT group.

The access he gave to press was restricted and did little to demonstrate he was anything more than a contributor/uploader (well after his June 15th apperance).

However, there was a ridiculous attempt to bamboozle a reporter to convince them of a connection between DCLeaks & Guccifer 2.0 via BadVolf (unable to actually demonstrate access and tries to use multimedia props as proof!)

He used Russian smileys because it's difficult to do a normal smiley on a Russian keyboard.

It's a lie that the ":)" smiley is difficult on Russian keyboards (SHIFT+6). - It should also be noted that he only ever used the Russian smiley twice and one of those instances was on his very first blog post! (The other was in a DM with Robbin Young) - This appears to be a forced action rather than habitual).

He referred to hacks as "deals" which proves he's a Russian. He did this throughout an interview in August 2016 with VICE. However, what's bizarre about it is that he referred to hacks as "hacks" and "breaches" at all times prior to that interview and at all times since. (It was clearly put on for the sake of the interview and was not something Guccifer 2.0 ever did habitually.)
He hacked the DNC & DCCC, he had to be state-sponsored GRU/FSB hacker.

His claims about breaches were debunked by ThreatConnect and none of his hacking claims were independently verifiable. - The only questions this really raises are:

- Why could he not give legitimate explanations?
- How did he really get DNC/DCCC documents?

Would 17 intelligence agencies lie? What makes your assessment better than theirs?

It was 3 agencies, they gave assessments that were dependent on flawed intel from private cyber-security firms, they never had direct access to the supposed crime scene and evidence.

Their assessments also predated the fingerprint fabrication efforts being reported and other evidence that has been uncovered in the last 6 months.

He tried to hurt the Democratic party so he must have been working for Trump, Russia, GRU/FSB, etc!

All of his leaks were mostly worthless and outdated junk that really posed little risk of harm to the party (and none of his leaks caused any damage to Clinton).

The DCCC docs, despite Nevins & G2's claims of "being worth millions" were pretty much all constructued using data available to both parties pushed through various document templates (to create a large volume of documents that would only really be of any use in a congressional race and weren't worth anything close to the lofty amounts suggested by Nevins & G2).

The TARP docs related to a scandal that was exposed 9 years ago and were unimpactful.

The HRC "opposition research" was not damaging, it was an objection handling guide that gave the official positive spin to be used to counter common criticisms of Clinton.

Exposing that the party considered Hillary the inevitable candidate at a point when Bernie was polling in single digits wasn't controversial.

The information showing fund-raisers and the amounts they'd raised would inevitably have to be disclosed so this in itself wasn't supposed to have been a controversy... it was only due to 4chan users noticing a correlation between ambassadorships and those listed that really caused any harm... it wasn't implied by Guccifer 2.0 when he 'leaked' the information.

The only really damaging document he released was in relation to Trump.