(Update 1st July 2012, I got a high 2.1 for these answers)
Was Lenin successful in his efforts to adapt Marxism to Russian conditions?
Lenin's attempts to adapt Marxism to the unique conditions of Russia were generally a failure, and those that can be considered a success were not "adaptations" in any meaningful sense of the word, but were on the contrary almost complete concessions to Narodnik ideology of peasant socialism.
To begin with, one of Lenin's main adaptations was the implementation of the peasantry into the revolution. Marx and Engels had entirely dismissed the peasants as part of the petite-bourgeoise and of no revolutionary capability. Lenin on the other hand called for a union between both agrarian and proletarian workers, an understandable move given that the peasantry roughly made up for 80 percent of the overall population at the time. Like the next adaptation to be discussed, this can largely be seen as a complete concession to the ideas of Alexander Herzen and the Narodniks who when agitating for an egalitarian revolution, had emphasised the role of the peasantry instead.
The next adaptation Lenin made was the role of the 'dictatorship of the proletariat'. Lenin recognised alongside Marx and Engels that the Paris commune of 1871 had resulted in failure due to the inability (or unwillingness) of the French Communards to use suitable measures to quell counter-revolutionary sentiments. Marx in particular however, had partly been in agreement with Bakunin and the anarchists that the workers had the potential for revolutionary spontaneity and were capable of maintaining their own revolution. Lenin however, saw politics as a constant process of conquest and therefore urged for an elite group of intelligentsia to not only give orders and command the revolution from the top down, but to use brutal repression when the need arose. This advocacy of terror was reminiscent of the tactics advocated by the populists and once again may be seen as a concession to the ideas of an ideology at odds with Marxism.
Although it may have been a revision of Marxist theory, one could argue that it was successful on practical grounds. Indeed, it was remarkable that the Bolsheviks were able to overpower both the Black and White armies and defend their revolution for as long as they did. Nevertheless, the extent to which violence was used must be looked upon as a moral outrage. A perfect example of the lengths to which terror was used is the Kronstadt incident which resulted the deaths of thousands in response to their call for political reform. On Marxist grounds it may immediately be seen as necessary to protect the revolution against reactionary activity however the vast majority of those killed in the Kronstadt uprisings were anarchists, social democrats and other groups that would have been fairly ideologically sympathetic. Clearly then, this was anti-populist and cannot be viewed as any kind of success through a Marxist lens.
Perhaps Lenin's greatest failure was his attempt to bypass the bourgeoise revolution. According to the Marxist analysis of history, in order for socialism to take place there must already be an established industrial base brought about via the period of capitalism. Russia's proletariat then, according to orthodox Marxist such as Martov had to wait for the bourgeoise anti-monarchist revolution of February 1917 to mature, long before they could even consider establishing a worker's state. Lenin however, noting the conservative nature of the bourgeoise that made up the February revolution (many of whom were former aristocrats and elites from the Tsarist years), and insisting that a world wide proletarian revolution was imminent believed that there was no time to wait for such a process and that the provisional government therefore was illegitimate.
Russia's ability to mature industrially without (according to orthodox Marxism) the necessary stage of capitalism taking place was seen as a false hope, and as a result the New Economic Policy (NEP) was implemented in 1921 which made multiple market based concessions in order to help her economy recover.
Ultimately then, I conclude that those few "successful" adaptations Lenin made were in actual fact complete revisions of Marxist theory, and in substance were revivals of the ideas of his second (only to Marx) ideological patron saint Alexander Herzen. The rest of which had extreme social costs, one of which lead to the greatest capitalist concession in Russian history.
Describe the main characteristics of the mixed economy of the NEP years (1921-1928) and account for why this system was abandoned?
The main characteristic of the New Economic Policy from 1921-1928 was the concession to a capitalist mode of production which was ultimately incompatible with the more revolutionary segments of the Russian population. This conflict would lead eventually to the disappearance of the NEP.
Firstly, one of the main features of the NEP was the Soviet government's decision to replace obligatory agricultural produce seizure with a light taxation. Rather than the peasant population being forced to give up their surplus produce, they were granted permission to maintain and trade it at their own leisure. This resulted in the reappearance of multiple marketplaces throughout the countryside, and also saw the reemergence of a handful of "well to do peasants" referred to as Kulaks. In order to boost grain output, peasants were also granted lengthy but temporary tenure over small plots of land. Although the land was formally "nationalised" in practice this was a revival of private property. This type of concession was at odds with the communist government which despite various economic reforms, remained increasingly illiberal in the political sphere.
In addition to the agrarian reforms, there were also various capitalist concessions in regards to Russia's industrial base. Many former owners and managers were granted their old factories, as their expertise was needed in order to bring Russia out of her industrial malaise. Those who benefited from the NEP reforms were referred to as "Nepman" and were viewed with increasing suspicion by the politburo. Even those who advocated the NEP such as Stalin and Bukharin agreed that it was only to be temporary measure until their knowledge and expertise had spread and the economy could be centrally planned efficiently.
Because of this, there were still various important sectors of the economy still controlled by the state (such as transport and heavy machinery) that worked alongside the new private sector. However, even these sectors made concessions of their own. In order to incentivise harder work and increase industrial output, rewards were granted to favourable workers such as furniture, cars and home appliances. This can be seen as a concession to the desire of self betterment and the profit motive, something Adam Smith had long since agreed was more effectively harnessed by the free market.
Economists such as Ludwig von Mises, F.A. Hayek and Eugene Bohm-Bawerk had long since defended interest and profits as necessary for determining the appropriate direction of resources. In addition, Mises and Hayek had argued that economic planning was impossible without the use of a medium of exchange to aid calculation. These prophecies were proved completely accurate in Russia, not only by the Soviet's decision to abandon the utopian aim to do away with money and restore the old currency (ruble), but also by the opening up of a network of banks in 1922. These banks were state owned, nevertheless they were a concession of the need for interest in aiding the distribution of wealth. Despite instability in the initial stages, hyperinflation was completely under control by the mid twenties.
The reforms made in agriculture successfully resulted in a return to prewar output by 1924-27, however the concessions also made it difficult to encourage the transport of grain to the inner cities to feed the growing urban population. The Russian proletariat desired egalitarianism and an equality of fortunes, naturally therefore it was viewed with resentment and hostility that the peasants were able to better themselves and make profit while they were experiencing shared food shortages.
These same sentiments were being increasingly mirrored within the politburo, and the initial plan to increase the size of Russia's industrial base by increasing the urban population was being undermined by the lack of trade from the peasantry to the cities.
Although it was intended for the partial privatisation of industry to be a temporary stage, and for the state sector to quickly replace the private sector once enough expertise had been gathered from the former managers and elites, the former continued to lag behind relentlessly. The "left" wing of the politburo increasingly recognised that these two opposing modes of production could not exist alongside one another forever and that one had to disappear for the former to survive. By the thirties, all remnants of the NEP had disappeared until the state eventually controlled 97 percent of the means of production.
To conclude, the NEP represented a consistent concession to market measures which lead to successful improvements to the economy. However, there was an increasingly instability in the pairing of communist and capitalist policies. Despite the successes in rebuilding the economy, the market reforms bred ideological hostility from the more orthodox sections of the politburo and the urban public which ultimately lead to its abolition.