I think the reason Deus Ex did it's voice acting so well was because they consciously planned it to be that complex. There were still things you couldn't do, like kill Gunther after the meeting with your brother (I tried, LAM's didn't hurt him and if you opened fire with a regular gun you had 20 or so Unatco troops shooting at you) or heck, even staying with Unatco instead of working with the Illuminati. But there were a lot of little choices that would effect the gameplay, which made it legendary. Wish more games would copy that.
On the flipside, I was surprised when playing through L4D2 at how many lines each character had. For example, when playing through the mall they see posters for Jimmy Gibbs Jr's stock car. On my first playthrough, Rochelle asked who he was and Ellis answered. The next time, Coach asked and Rochelle answered. That really added a lot, in my opinion, since it made multiple playthroughs have less of a "heard it already, let's shoot some zombies" feel to it.
If you want to see a game with bad voice acting, try playing Musashi Samurai Legend, or the second Ape Escape. On second thought, don't, nobody should have to play those. The kid in Musashi hammed it up (which was a shame, as it was a spirit sequel to Brave Fencer Musashi, which had 90% good voice acting and was very fun to play), and most of the other characters were only passable. Ape Escape 2 used the voice actors for Pokemon in their english translation, which almost worked until the original Spike showed up... voiced by the same guy who did Yugi. Between the worse gameplay, the ridiculous plot (time travel made more sense than 100 or so little chimps taking over the world), and the horrible voice acting, I seriously wish they had never even attempted a sequel.