The probably deliberate shootdown of the Su-24 over Syria by Turkey has underlined that Russia in Syria is vulnerable, despite Russia's great power. Russia is for one hobbled by geography, and Russia is also vulnerable to relative local superiority of forces of opposing nations like Turkey.
Recent events have highlighted Russia's problem of supply, and its problem of supply in light of attrition, and attempts to open up new fronts to distract Russia from its main objectives.
♦ Threat to Russian lines of communication
There are basically two ways for Russia to supply its operations in Syria - one is by sea, or more precisely, from the Black Sea through the Dardanelles, the Mediterrean Sea - and the other by air and land over the Caucasus region (Azerbaidjan), through Iran and then Iraq. All these supply lines are long, and run east or west around Turkey which sits in the middle - an obstacle when it wants to.
Under the Montreux convention the Bosporus is open to Russian naval vessels in peacetime. In times of war the Turks could close it to Russia - bottling them up in the Black Sea and/or trapping them in the Mediterranean. Replenishment by sea or reinforcement to the Mediterranean would then have to come from the Baltics, which would take weeks, entirely through NATO controlled waters.
Had the shootdown resulted in the successful invocation of NATO's collective defence clause under Art.5 the resulting state of war would have allowed Turkey block the Dardanelles for all Russian naval vessels.
♦ Crimean diversions - Tartars!
Crimea is Russia's main port of supply and HQ of the Black Sea Fleet - most of the Russian reinforcements to Syria originated there. The electricity lines that run from Ukraine into crime have been sabotaged by Crimean Tartars, who also obstruct reconstruction.
The Crimea blackout also left, as collateral damage, parts of south-eastern Ukraine without electricity - apparently worth it. The incident shows the weakness of Ukraine's central government in face of rabidly rightist forces, since apparently the saboteurs enjoy political cover. For Putin, the incident is a distraction - a small defeat which may hurt him in the polls, hands him yet another problem to solve, at a cost - and it gives a small petty victory to the Ukies, the Tartars (and their probable Turkish backers), gets Crimea back into the news, and it hurts Russian operations at least somewhat.
Crimean tartars were subjected to brutal persecution under Stalin for their real (there was a Tartar Waffen SS unit) and alleged cooperation with the German invaders. Turkey also sees them as 'unredeemed' Turks. Their cause resonates in Turkey also, as an estimated 5 million Crimea Tartars (who went there since the days of Catherine the Great) are said to live in the diaspora in Turkey.
I had Turks tell me to my face that, as far as they are concerned, Crimea is rightfully Turkish. Turkey lost Crimea 232 years ago when Catherine the Great annexed it. It's been a while since.