Lots of things can disrupt SOW:
1. Someone's tired.
2. DM had a rough day at work.
3. Cell phones (yeah, it's been mentioned, but, it deserves it twice. I feel your pain)
4. Pizza delivery.
5. Children/spouse/parents/random strangers in the gaming environment.
6. Internet problems when you run online games. (GRRRRRRR)
7. Dropping dice.
8. Getting tongue tied or speaking in Spoonerisms.
9. Cliche's
10. Gas.
11. Players showing up late.
That's enough for now.
All quite valid, but let's say you and the group have overcome these obstacles for the night...you've got decent immersion and buy-in from the players, you yourself are on form as DM, things are rockin' right along - in other words, an all-round stellar session.
Why is it still so hard to find that sense of wonder?
To me, the biggest wonder-wrecker is
too much knowledge:
== knowledge of rules, of what's "behind the curtain", and of the math involved;
== knowledge of items;
== knowledge of the game world (given a choice I'll play in a homebrew world every time, only because it's new and thus waiting to be discovered);
== knowledge of the rules beyond just how to play, of rules tricks, loopholes, and broken combos;
== knowledge of monsters;
== knowledge of the DM, though some are good enough to reinvent themselves and thus keep it fresh.
With work, a DM can mitigate the first, second, fourth and fifth of these by (in order) shaking up the math, replacing known item lists with unseen homebrew lists, closing loopholes/banning combos, and reskinning monsters; with the proviso that step 2 has to be to make this information harder to come by in game (eample: use 1e item identification rules rather than 4e).
For my current campaign, I tried to rename quite a few of the standard monsters just to shake it up - underneath they were the same monsters, but the players (some of whom are almost 30-year veterans) found themselves to ask what a "Grash" or a Quitch" looked and acted like - even though they are merely Orc and Kobold renamed. Now, 2.5 years in, they all know what's what; but it worked really well for a while.
Lan-"a little knowledge is a bad thing; a lot, worse"-efan