Well, it's not a representative selection of sources. Most mythical texts that refer to the afterlife focus on the punishment of evildoers like Tantalos and Sisyphos. Since the gods supposedly ensure that they get their just desserts in the end, naturally that picture of the afterlife is pretty grim. Those who work on the idea of death in ancient Greek thought -- thanatologists -- consistently come to pretty much the opposite conclusion about death: traditional beliefs about death were for the benefit of the living, not for the dead, and dying a good death was enormously more important than having a comfortable situation in the afterlife.

If you leave the fate of evildoers aside, we don't really have a huge amount of detail about what was believed about the afterlife. It certainly had its downsides: it was dark (the Sun shines only on the living), and the dead weren't allowed to leave; but that's pretty much it. We have depictions of legendary heroes in Iliad book 23 (Patroklos' ghost); Odyssey books 4 (Menelaos' destiny in the Elysian fields), 11 (the psychagogia by the water), and 24 (the ghosts of dead heroes in the Underworld); the Hesiodic Theogony (Kerberos stops the dead returning to the world of the living). For them the house of Hades is a mixed bag. On the one hand, they're not allowed to leave, and they're not too happy about that; on the other hand, they get to hang out, chat, and generally mess around in the asphodel fields.

On "mindless wandering": the picture across the various sources is inconsistent. Sometimes the spirits of the dead are witless flitting shades without thought or sensation; sometimes they're basically like living people, apart from the fact that they're incorporeal. The "witless shades" picture is the less common one, and is usually overruled by the second one; it's possible that it may be an earlier belief. Even within Odyssey 11, the narrative swaps back and forth between the two options: at the start, the dead have to receive a blood offering before they acquire cognitive capabilities, but the narrator kind of forgets about this after a while. Iliad 23 also presents an inconsistent picture: at one point Achilleus refers to the witless dead, but Patroklos' ghost is pretty coherent and reports that other ghosts acted to keep him away from Achilleus. Later accounts almost exclusively adopted the second option, as in the satirical version of the afterlife in Aristophanes' Frogs, which depicts the house of Hades as a replica of Athens, with its own bureaucratic and political machinery; and in Plato, where dead people get to hang out together and chat, and the immortal souls of virtuous people are rewarded handsomely after death (in a very philosophical way, naturally!).

So sometimes being dead looks like being a witless ghost, but mostly it looks like wanting to go back to the world of the living but not being allowed to. The emphasis tends not to be on the condition of being dead, anyway; it's more on the transition from life to death. Depictions of that transition vary as well, and death is never a nice experience, but generally speaking the daimones that guide spirits on their way after death -- especially Hermes and Charon -- are shown as benevolent, reassuring figures.

Interest in the condition of being dead seems to have been the main appeal of what we now call the "mystery religions". These were supra-regional cults that were not always closely linked to the cult of the Olympians. We have intimations of salvation after death for initiates, hinted at especially in the Eleusinian Hymn to Demeter and in Orphic texts like the the Orphic gold tablets and the Derveni papyrus. They're necessarily very vague, as there were strong taboos against revealing the hidden knowledge of mystery religions, but the gist is pretty clear: initiates would be rewarded after death, non-initiates would suffer. Here's what the Hymn to Demeter says:

...[Demeter] went to the lawgiver kings,
Triptolemos and horse-goading Diokles,
strong Eumolpos and Keleos leader of hosts, and taught them
the sacred service, and showed the beautiful mysteries
to Triptolemos, Polyxenos, and also Diokles --
the solemn mystereies which one cannot depart from or enquire about
or broadcast, for great awe of the gods restrains us from speaking.
Blessed is he of men on earth who has beheld them,
whereas he that is uninitiated in the rites, or he that has had no part
in them, never enjoys a similar lot down in the musty dark when he is dead.

Here's a sampling from the Orphic gold tablets:

...I also claim that I am of your blessed race.
Recompense I have paid on account of deeds not just;
Either Fate mastered me or the thunderer flinging the lightning bolt.
Now I come, a suppliant, to holy Phersephoneia,
That she, gracious, may send me to the seats of the blessed.

But when the soul leaves the light of the sun,
go straight to the right, having kept watch on all things very well.
Hail, you having experienced the experience you had not experienced before.
A god you have become from a man. A kid you fell into milk.
Hail, hail; make your way to the right,
the sacred meadows and groves of Phersephoneia.

...They will ask you, with sharp minds,
why you are seeking in the shadowy gloom of Hades.
Say: "I am the child of Earth and starry Heaven;
I am parched with thirst and I perish; but give me quickly
refreshing water to drink from the lake of Memory."
And then they will speak to the underworld ruler,
and then they will give you to drink from the lake of Memory,
and you too, having drunk, will go along the sacred road that the
other famed initiates and Bacchics travel.

So there you go. Looks pretty optimistic to me... well, as long as you're an initiate, that is.