I voted for "no surprises, thank you" and I expected to be the only one but no... now THIS is a surprise!
Now to detail my brutal answer

I don't like surprises indeed because I like the planning part, especially if this involves maps or charts of any type

. And surprises have the bad habit to turn a smart planning void.
A few words about the suggested sources of surprised:
Real weather should be a nice feature but transitions between areas with different weather are anything but nice. It is just too easy to get into a stall or overspeed with no warning at all, and this is annoying, especially for PP flights.
However, as fair weather is rather dull, I mostly fly using one of the standard themes in FS ("gray and rainy" at this very moment). This provides realistic weather without unrealistic changes.
Technical failures... possibly the only thing I hate more than surprises. As an engineer, of course.
Busy AI skies... I tried this and I don't find it surprising, but rather stupid. Yesterday an AI aircraft (with the blessing of AI controller) crossed the very runway I was cleared to take off from just in front of me, it was a VERY near miss. I don't know whether AI is much better in FSX; if it is, it would be a serious reason for me to switch.
FS Passengers... Heard a lot about it looks like something I could try... one day.
And finally, I think there is one important "surprise factor" missing from the list, and I mean online flying. While online I don't like surprises either - but at least I tolerate them!
Andrei