Hybrid annotation techniques have emerged as a promising approach to carry out named entity recognition on noisy microposts. In this paper, we identify a set of content and crowdsourcing-related features (number and type of entities in a post, average length and sentiment of tweets, composition of skipped tweets, average time spent to complete the tasks, and interaction with the user interface) and analyse their impact on correct and incorrect human annotations. We then carried out further studies on the impact of extended annotation instructions and disambiguation guidelines on the factors listed above. This was all done using CrowdFlower and a simple, custom built gamified NER tool on three datasets from related literature and a fourth newly annotated corpus. Our findings show that crowd workers correctly annotate shorter tweets with fewer entities, while they skip (or wrongly annotate) longer tweets with more entities. Workers are also adept at recognising people and locations, while they have difficulties in identifying organisations and miscellaneous entities which they skip (or wrongly annotate). Finally, detailed guidelines do not necessarily lead to improved annotation quality. We expect these findings to lead to the design of more advanced NER pipelines, informing the way in which tweets are chosen to be outsourced to automatic tools, crowdsourced workers and nichesourced experts. Experimental results are published as JSON-LD for further use.