The basic attribute of Marathi people (Marathi mentality) is that typically they are not known to be businessmen. Even at the zenith of their imperial expansion, Peshwa Balajirao had to invite traders from Marwad to invest in Maharashtra start their business centres. The entrepreneurial attitude of Punjabis, Marwadis, Udupis, Gujaratis and few others is not typically seen in Marathis.
Having said that, the class which is voter-base of Raj Thackeray (RT) is the young middle class urban maharashtrians. Thus, most of them are educated. The problem with North indians (Bhaiyya as they are called) is somewhat different from the problem with South Indians (Madrasi as they are called) which BT had in 70's.
South Indians being well versed in english (Better than marathis) were easily getting higher positions in government jobs in late 60's. The Udupi lobby in Mumbai was dominating the hotel business and food industry. To disarm and eradicate communism from MH, INC supported and propped up Bal Thackeray (BT). BT must be given credit of eradicating the communism from MH for good.
BT invented "Wada-Paav" and "Jhunka-Bhaakar" and made available loans for maharashtrians to compete with outsiders in hotel businesses and other low end businesses. Marathis never had any monetary capability to enter the real-estate business of Mumbai and Bollywood. This is dominated by cash rich Sindhi and Punjabi lobby, most of which were the descendants of Shikarpur traders who liquidated their huge assets in Pakistan before coming to Mumbai after partition. Along with Marwadis and Gujaratis, these people formed the new elite of Mumbai dominating the ownership in FMCG, jewellery and Textile industry. Marathis never had chance of competing with this class. Their niche was low-end business, service, arts. Of course for uneducated, the unskilled labour-work was always present.
This mentality however was first manifested in history of independent India during Samyukta Maharashtra movement. One should understand that the linguistic pride in Marathi is not like South Indian linguistic pride shown by Tamils. This pride is more about Maratha empire. The popular metaphors in all the pride-instilling songs of that era (Samyukta Maharashtra movement and later BT) refer to Maratha horses drinking waters of Yamuna (conquest of Delhi) and Sindhu (Attock). The fundamentals behind Marathi linguistic movement and pride are based on lost empire and power and not primarily love for language. When British formalized their rule in 1858, the biggest losers in India were Marathas. This is somehow not forgotten in subconscious minds of Maharashtrians.
In current times, the threat to the social niche of marathi people in urban MH is from people of UP and Bihar. This apparent threat different from previously perceived South Indian threat in following ways..
1. South indian class which migrated to Mumbai and MH was not linguistically imposing. The Southies show much more readiness to speak the languages which Marathis understand (Marathi OR Hindi). This readiness in attitude is NOT SHOWN by North Indians from these two states. South Indians have totally different script and language, their effort to learn hindi and marathi was appreciated and acknowledged by Marathi people. From hindi speaking people, lot more is expected and it is justified. It is not really difficult to learn marathi for hindi speaker, especially since the script is same, most of the words are same. The general disinterest shown by UP-Bihar people in learning local language and customs ignites the spark and this has been the complaint of many other states (including Punjab, Himachal, Haryana, Rajasthan, South and North-east).
2. The socio-economic and political conditions of the UP-Bihar is worse than MH and rest of India. This is mostly due to the incompetent rulers which have been ruling that region for 3 to 4 decades now. The scale of migration of unskilled labour from these two states into rest of India justifies the usage of term "outrageous". The cultural arrogance coupled with fact and perception that Gangetic plains suck up the money generated by other provinces (i.e. MH) further exacerbate the tensions.
3. What is puzzling the most about RT is that the voter-base which supports him is that of educated middle class youth from urban centres. The migrants from north are mostly illiterate unskilled labourers and do the work and jobs which marathis typically have shied away from. The voter base of RT is mostly white-collared youth OR youth aspiring to do white collar jobs. This fact puzzles me the most, who is the real target of RT ?. In case of BT, it was well defined and actual.
4. The issue of importing unskilled labourers from Gangetic plains for jobs in MH which can be done by local unskilled labourers is relevant and pertinent. But Maharashtrian rural unskilled labourers (the direct beneficiaries of this RT policy) are not the voter-base of RT.
5. The urban issues raised by RT are pertinent and very well taken by marathi people - sign-boards in local language, advertisements for railway jobs in MH to be published in Marathi papers and not in Hindi papers published in UP, the Jet-airways crisis and his timely threat which was immediately effective.
6. One more reason which fuels the anger towards North Indians is the leaders who vouch for them in Mumbai and MH. The leaders like Abu Azmi, Amar Singh, Mullah Mulayam etc are totally incompetent and corrupt. Sanjay Nirupam was a Shiv-sena man and used to participate actively in "Bhaiyya-Bashing" before he was denied ticket. Hence he entered INC and started shouting against BT and RT. The locals know about his reputation. Kripashankar singh is targeted but much less; the fact that he can speak decent marathi helps.
7. Another angle is that of underworld and terrorism. The UP, especially the doab is the region from where abu salem and many other dons, thugs and terrorists have come. Muslims from Azamgarh is very popular bashing stick for RT and BT; same goes for countless illegal Bangladeshi's. The leaders who choose to stand for UPaites in Mumbai and MH are of shady character, hence the anger is further multiplied. Nitish Kumar is not criticized by RT and BT as much as Azamgarhi Abu Azmi, Mulayam, Amar singh and lalu yadav are.
8. Over all, in my opinion, this is more about resurgence and outlet of some hidden subconscious anger in minds of Marathi people and Maharashtrians that they are left behind in the race for power-struggle of India's heartland. After B.R.Ambedkar there is no big Marathi leader who has important say in Delhi power circle. Pawar is powerful but has bad image even in MH outside his stronghold; Pramod Mahajan was promising, but is no more. The urge seen in Saatvahans, Western Chalukyas, Rashtrakootas and Marathas is exactly this same urge which is manifesting itself in form of BT and RT. This urge is to assert themselves and participate decisively in politics of Indo-Gangetic plains.
9. Since this urge, coupled with anger of lost power/importance at the hands of north (delhi - symbolized by mughal emperor) is behind emergence of this mentality, I don't think this movement can remain aloof from hindutva cause or it can divide and enervate it. Any spark in gangetic plains which calls for rallying of hindus, this region and these people will be first to jump in for that cause. BT did the same during and after 1992 riots.
P.S - Please read the comments of this article as well as other articles. I have written a elaborate post-script comparing the Tamil linguistic movement and Marathi linguistic movement which is pertinent. Also inputs of other readers are priceless..