1. The West
The West is not as cowardly as it appears, and will go forward with sanctions against Russia and with admitting Ukraine to the EU and then to NATO. There are many things the West will pretend not to see, but invading a major country situated in Europe is not one of them. Putin's actions are offending Western sensibilities, and that's something that will make the West overcome its cowardice.
2. Russians in Ukraine
Most Russians in Eastern Ukraine are not interested in joining Russia (according to the recent polls). In fact, some are irked by the invasion. Nothing consolidates a nation more than an unprovoked foreign invasion. Especially right after a national anti-corruption revolution. So, instead of building up influence and support in Eastern Ukraine, Putin is going to be gradually losing it. (A lot more so when a Ukrainian passport will mean ease of travel to the West and ease of doing business, while a Russian one will mean the opposite.)
3. Russian economy
Russian economy depends on oil and gas exports and on foreign investments, and will suffer a downturn. Combined total of foreign credit lines to Russian companies is close to 1 trillion USD, about half of Russian GDP. Falling Rouble and declining economy will eventually cause a great unrest of lower classes, Putin's main base. Occupation of Crimea and international sanctions will continue to lay a heavy toll on Russian economy.
4. Unrest in Crimea
Crimean Tatars will not be easily pacified. Ukraine will not agree to let Crimea go. Crimea will become a destabilizing influence on Russia, much like Chechnya had been, but on a different scale (proximity of Ukraine and Turkey, their size and influence, 40% of non-Russian population in Crimea, access to the Black Sea, location in Europe, and many other factors will play against Putin's troops here.)
5. Crimea as a burden
Crimean economy, dependent on tourism, will suffer from absence of non-Russian tourists (said to be about 70% of all tourists), and its 2 million inhabitants will essentially become an added burden on Russia. Crimea also has the highest percentage of retired people compared to any region of Ukraine and Russia. They all will expect their pensions to be paid by Russia. The need to build a bridge, roads, renew infrastructure and pay for water and electric energy currently supplied by Ukraine will cost extra tens of billions of dollars.
6. Russia's gas and oil powers
Russia does not have a monopoly on gas and oil, and its actions in Ukraine are precipitating the search for alternative suppliers and for alternative energy sources. As time goes by, Russia does not cease to provide reasons for the West to do so as fast as possible. Russia is now likely (probably in just a few years) to find itself in a somewhat similar situation to that of Iran - with its overabundance of gas and oil, but not much demand for it.
7. World politics is not Olympic games
Winning in Sochi was a great achievement for Putin and Russia, and created a sense of national pride, power and prestige. However, German medal count in 1936 Berlin Olympics was even more impressive (Germany won every third medal), and led to a similar exhilaration... we all know where Germany ended up. World's politics function in very complex ways, and even when a country with the strongest military producing over 20% of world's GDP neglects that complex mechanism, it hurts itself. Let alone when a country produces less than 2% and has rather inept military, and yet decides to act unilaterally.
8. Russia's allies
No major country supported annexations of Abkhazia and Ossetia, and none will support that of Crimea. Alexander III's apocryphal "Russia has no allies but its army and fleet" will finally ring true. And that would be Russia's outdated fleet and Russia's weak army. It's unlikely that the West will send troops to fight Russia even if Putin's tanks will cross into Eastern Ukraine, however, even holding on to Crimea will be a costly and tough undertaking.
9. Russians and Ukrainians
Russians and Ukrainians are two very brotherly and similar people sharing as much common history as the UK and the US. By annexing Crimea, Putin is pushing Ukraine away much further away from Russia than it would have ever gone otherwise. Yanukovich, a pro-Russian candidate, had won the last presidential election in Ukraine, and pro-Russian parties got over 40% of vote in the Parliament. But after the Russian invasion in Crimea - many years will pass before a pro-Russia candidate will have good chances to be elected Ukraine's president.
10. Russian revolution
Russian revolutions usually followed a war. The Revolution of 1905 was precipitated by the 1904-1905 Russia-Japanese War. The Revolutions of 1917 happened during the World War I. And 1979-1989 Soviet-Afghan War had a tremendous impact on the USSR. The population of Russia currently overwhelmingly supports Putin. In fact, his approval ratings are at the highest level for years, and will soon reach the record level of all times. However, Russia went from popular support to the Emperor and his War in 1914, to a popular uprising in 1917.