There is definitely some awkwardness involved in the Hide/MS and Listen/Spot merge.
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I was on the side that wanted the skills to remain separate. But I've pretty much come around on that not because I changed my mind on the mechanics so much as I am forced to admit that so many characters always had the same number of ranks in both Hide and MS, that blending them ends up making no difference.
That said, I strongly think it is the DM's responsibility to keep the practical distinctions in mind. Hiding behind a tree at night and creeping across dried leaves are both Sneak vs. Perception checks now. But the DM should consider modifiers based on the interaction. According to a strict reading of TB the guy trying to quietly creep across the dry leaves gets vastly better at being quiet if he is invisible. Obviously, that is silly. In that case the Sneak vs. Perception check should have no modifier whatsoever resulting from the invisibility. Now, assuming the guard hears you, you are still invisible. He knows he heard something, but he probably doesn't know what. And another check is in order, and for this one the +20 bonus is appropriate.
I suppose the argument may be that two rolls defeats the purpose of merging the skills. But, to me, this circumstance needs the two rolls to provide a quality resolution. In some circumstances under old D&D there would be cases in which sneaking across a room would require both a hide and move silent in order to remain undetected. I'm fine with just using one master roll for those type scenarios. But this is different.
For hiding behind the tree, yeah the rogue is better than the wizard, but the +40 bonus makes the rogue's +55 not really meaningfully better than the wizard's +42 unless you have some really intense Spot checks coming. and that makes sense. You are standing still and invisible. This qualifies as hard to spot.
The bottom line is, just because you only have Sneak and Perception as skills, you don't get away from situations that demand Hide/Move Silently/Spot/Listen. You just use a simplified system for establishing the modifiers for the check.
Back on the real topic: Fenris said
Moving them to skill bonuses is great. Invisibility grants a +10 to Stealth,...
Maybe I'm reading to much into that. But taken literally he is saying Invisibility should "move" from where it is now, to a simple mechanical skill modifier. To that idea, I am strongly opposed. Invisibility should first and foremost make the target invisible. Adding in a clear guideline for modifying skills (Stealth) as appropriate makes total sense. But only as a supplement to the main rule.
If you just go with modifiers, then you get into silly situation that compare to 3.5 Darkness. You have a spell that doesn't do what it says. I honestly found Darkness to be a perfectly sound and defensible 2nd level spell. It just had the wrong name.
If you want a spell that gives a Stealth modifier, that sounds cool. Call it Rouge's Obscurement and stat it up.
