Factors beyond Aviation
1. Though it seems that most here have discounted the possibility of a surface-to-air missile strike, I want to underline that it is extremely unlikely. To covertly deploy the necessary equipment, crew and logistics in the Sinai, sufficient to strike an airliner at cruise altitude, would be a very tough undertaking. To single out a Russian flight among the substantial air traffic over the Sinai would be more difficult still. And as someone already observed, Israel likely would have detected and reported a SAM launch.
2. Major Russian air carriers seem to (generally) operate in accordance with international safety standards. However, the post-2000 record of smaller operators in the Russian Federation is scary.
3. The political effect of this crash may be more potent than any aviation incident in Russian political history.
3a. If investigation discloses evidence that deficiencies in quality and government oversight contributed to the crash, this will (I predict) strike a particularly raw nerve among the Russian people, and their confidence that government (or any institution at all) is protecting them. Russians often suffer from avoidable calamities (deadly hospital fires, to give one example) that would be really shocking in more prosperous countries.
Egypt is an enormously popular vacation spot for Russians. The list of countries you can easily enter on a Russian passport is a short one, and most of those are nobody's idea of a vacation spot. Accordingly, millions of Russians can easily visualize their own family as one of those returning on the doomed flight from a sunny holiday. My personal observation is that in Russian culture, events like this tend to be received rather more sentimentally and less analytically than in the West ... this one really hurts.
3b. If investigation discloses that a smuggled IED caused the crash, then this will react with Russia's abrupt initiation of attacks in Syria. I'm sure most Russians didn't see the Syria campaign coming, and there were no crowds on the streets in Moscow demanding the Kremlin to do such a thing.
Part of the political function of the Syrian campaign is to distract the populace from the dead-end of Russia's aggression against Ukraine. If you recall Orwell's "two minutes hate," and the instantaneous switch between war with Eurasia and war with Eastasia, you will have some picture of how the Kremlin communicates to the Russian people about its military adventures.
If the surprise -- and unasked-for -- gift of Russia killing people in Syria comes at the price of the worst air catastrophe in the history of Russian aviation, then the Kremlin will have an acute challenge of image management.
4. In light of my point 3, although Russia's air safety investigation arm (a commission of IAC/MAK) has a good reputation and track record, it could face unprecedented political pressure in the investigation of this accident. Russia is not an exotic variant of the West ... things are really different there.