While Valve may have opened the gates to the public for their ARTS title Dota 2, the testing still continues with the addition of a new hero to the ranks, Abaddon.

When the infant Abaddon was bathed in the Font, they say something went awry. In the child's eyes there flared a light of comprehension that startled all present and set the sacerdotes to whispering. He was raised with every expectation of following the path all scions of Avernus took--to train in war, that in times of need he might lead the family's army in defense of the ancestral lands. But Abaddon was always one apart. Where others trained with weapons, he bent himself to meditation in the presence of the mist.
He drank deep from the vapors that welled from the Font, learning to blend his spirit with the potency that flowed from far beneath the House; he became a creature of the black mist. There was bitterness within the House Avernus--elders and young alike accusing him of neglecting his responsibilities. But all such accusations stopped when Abaddon rode into battle, and they saw how the powers of the mist had given him mastery over life and death beyond those of any lord the House had ever known.

The The Lord of Avernus is described as a very versatile melee strength Hero. The in-depth Dota 2 wiki reads that Abaddon can 'fulfill various roles due to his pool of skills which include a heal that can also be used as a nuke and a shield which makes the wearer invincible as long as it's active. He is most played as a support, although he can also be used as a semi-carry or even a tank due to his unique ultimate.'
Alongside the new hero introduction, the Dota 2 test client patch brings a number of gameplay changes, but also most notably the addition of a Linux and Mac OS X client. Currently testing is underway on the clients so if you do want to participate in the early testing action it is recommended that you swing by the Dev forums for more information, including a number of bug support threads.
Those wanting to the full patch notes to gloss over who is now the most overpowered and who has been nerfed merciless can find them here.