I'll try to explain some of the key ideas of the Left Opposition and the differences with Stalinism in theory and practice but most of this is gone into more detail in the
Platform of the Joint Opposition by Trotsky and others,
The Revolution Betrayed by Trotsky,
From Lenin to Stalin by Victor Serge,
Platform of the Workers' Opposition by Alexandra Kollontai and others and
Nine Years of Struggle of the Left Opposition by Max Shachtman.
The programme of the Left Opposition put forward a position of collectivisation of a completely different character to Stalinism. The Left Opposition, as early as 1923, before Lenin had died, had identified a growing strata amongst the peasant population of rich peasants (kulaks) who employed and exploited poorer landless peasant workers due to the NEP's concessions that stabilised the economy somewhat but allowed these rich peasants to benefit and accumulate wealth at the expense of other peasants. The Left Opposition suggested a steady build up of a campaign to convince the poorer peasants to willingly enter into collective farms through tax breaks, subsidies and access to modern agricultural technologies. This is opposed to Stalin's position of effectively supporting the kulaks (they ran a campaign that employed slogans encouraging peasants to get richer) in order to undermine the Left Opposition and then utterly reversing that position once all the Left Opposition had been expelled or imprisoned and therefore enacting collectivisation without any sort of groundwork laid, thus having to deal with a stronger kulak population and greater discontent (peasants even killed livestock enmasse in defiance).
The Five Year plans first proposed by the Left Opposition, to promote industrialisation and plug the gap between the demand for industrial and consumer goods from the peasants, were different in character as well. A tax on the kulaks, the richest peasants with the most land, was proposed to help pay for a series of hydro-electric dams and other power plants in order to supply a growing industry and the cities built up around them with electricity. In 1926, 56% of all surplus grain was in the hands of only 6% of the peasant population and it was this strata of peasants who the Left Opposition would tax in order to fund the development of industry. In contrast, Stalinism simply printed money with the number of bank notes in circulation during the first plan raising from 1.7 billion to 5.5 billion and then to 8.4 billion in the second five year plan - this also threw the price of grain and other goods into flux forcing even more grain requisitions.
The Left Opposition desired greatly to reintroduce party democracy of an honest and healthy variety. They wanted every party member or group of party members to be able to debate their positions and freely use the party press to put forward platforms and ideas to be discussed. The Left Opposition also wanted to increase the number of workers and poorer peasants introduced into the party as opposed to civil servants and middle/rich peasants, whose numbers had been increasing. They wanted to reconstruct the Central Committee to be less associated with the bureaucratic civil servant layers and instead change the composition to involve more workers and poorer peasants. The soviets in general were to be revived as organs of democratic proletarian power in the cities and through evidence of deeds draw in the wider layers of the masses to the programme to build socialism.
There's more I could go into but I don't have the time and can't be bothered. Serge in
From Lenin to Stalin introduces some of the key changes in the character of the party and the state that he and the Left Opposition had identified and rallied against that I'll transcribe now:
Everything has Changed.
The aims: from international social revolution to socialism in one country.
The political system: from workers' democracy of the soviets, to the dictatorship of the general secretariat, the functionaries and the GPU.
The party: from the organisation, free in its life and thought and freely submitting to discipline, of revolutionary Marxists to the hierarchy of bureaus, to the passive obedience of careerists.
The Third International: from a mighty organisation of propaganda and struggle to the opportunistic servility of the Central Committees appointed for the purpose of approving everything, without shame or nausea.
etc etc. He identifies more problems and developments that fundamentally altered the character of USSR but I can't be bothered to type out more. Hope this gives a hint of what might have been...