Generally, I've not found Marvel's D-T-DVD stuff released so far to be that impressive, but these both really worked, in terms of what they set out to do.
Starting out, I'd say that the Hulk is really just kind of there in both stories; he's worked into the plot, but in both cases the writers basically hijacked the initial idea (Hulk vs. Wolverine, Hulk vs. Thor) to tell some other story. In Wolverine it's "finally, we get to do an R-rated version of Weapon X", and in Thor it's "finally, we get to cram in absolutely every aspect of the Thor mythos that we have any interest in onto animation" (which, given that there's both an animated series and a second D-T-DVD which is trailed here, may have been a bit unnecessary, but it's still good).
Part I: Hulk vs. Wolverine
Basically a big fight scene. Wolverine is called in by Department H to deal with a rampaging Hulk, who has seemingly destroyed an entire town along the US-Canadian border. Wolverine tracks the Hulk down (not really knowing what he is at this point), fights him, and basically fights to a draw (this is the 70s-era version that Wolverine was created to fight), and then are captured by Weapon X (which apparently isn't associated with Department H here). This story is tied into the Wolverine and the X-Men cartoon, so Wolverine, Banner, Sabretooth, and Professor Thornton all have the same voice actors; also here are Lady Deathstrike, Omega Red, and Deadpool. Thornton wants Wolverine back in the program, but Deathstrike and Sabretooth would rather kill him, so they stab Thornton and, well, try to kill him. Big fight, etc. The attraction here is seeing "claw people" (as Hulk calls them) stab each other a lot with blood and stuff; Deadpool's quite funny.
Part II: Hulk vs. Thor
Starting out, I'll say that fans of Thor who were expecting an epic throwdown between the two will be disappointed here; this might more accurately be titled "Hulk walks all over Thor, and so Thor has to find some other way of stopping the Hulk". I personally have never found the uber-godmoder Hulk all that interesting, but it's an established thing, so there you go; getting past that, the story's pretty good, and if you like Thor's world pretty everybody is here.
The story takes place during the yearly Odinsleep, when the All-Father rests and all the villains try to conquer Asgard because Odin is absurdly powerful and they've only got a chance when he's not there. Thor and co. beat up all comers, as usual (Thor's getting rather depressed by this endless cycle). Loki and Enchantress capture Bruce Banner, and turn him into the Hulk; Amora then pulls Banner out the Hulk (and Loki kills him), leaving only the incarnate rage (see Hulk #300 in the early 80s), with Loki pulling the strings. However, the rage becomes too much and breaks Loki's control, and Rage-Hulk is loosed and starts destroying everything. He beats up the Warriors Three and Balder, then pummels Thor to death (to Loki's delight), and heads off towards the city; Hela is about to claim Thor's soul, but Enchantress intervenes and brings him back to life, since her whole motivation for helping Loki was she was angry that Thor chose Sif over her. Thor and Enchantress capture Loki, and Thor makes Loki go with him to Hel to reclaim Banner to put him back in the Hulk, and sends Enchantress to help Sif, who's the End Boss guarding Odin. So Thor and Loki journey thither, while Hulk punches his way through like a bajillion random guards, Brunnhilde and the other Valkyries, etc. on his way there.
As a tour of the Nine Worlds, it packs quite a lot into 40+ minutes (it's the longer of the two); both of these stories just drop you into an established world, which is refreshingly different from how a lot of these films do things; you pick up all the necessary information as you go along.
