Games with a purpose (GWAPs) are increasingly used in audio-visual collections as a mechanism for annotating videos through tagging. One such GWAP is Waisda?, a video labeling game where players tag streaming video and win points by reaching consensus on tags with other players. The open-ended and unconstrained manner of tagging in the fast-paced setting of the game has fundamental impact on the resulting tags. Consequently, the Waisda? tags predominately describe visual objects and rarely refer to the topics of the videos. In a previous study [ECIR 2013, 50-61], Gligorov at al. showed that the Waisda? tags are effective in finding video segments that depict a specific entity (person, object, etc.) of interest. This study evaluates the performance of the game tags for retrieval of videos that are about a given topic. To this end, we setup a Cranfield-style experiment for which we use an evaluation dataset that consists of: (i) a collection of videos tagged in Waisda?, (ii) a set of queries derived from real-life query logs, and (iii) relevance judgements. While we reuse the collection of videos and the set of queries compiled by Gligorov at al., we designed the set of relevance judgments specifically for this study. The novelty aspects of this paper are as follows. Our results demonstrate that the raw, unprocessed game tags are not well suited for retrieving videos based on topic. We perform a qualitative analysis of the search results which reveals that this is mainly caused by the presence of general tags which refer to visual objects unrelated to the topics of the video. Thus, we characterize the quality of the game tags as topical annotations in order to detect and filter out the non-topical ones. We explore several features of the game tags which could serve as an indication of their quality as topical descriptors. Our results show that after filtering, the game tags perform equally well compared to the manually crafted metadata when it comes to accessing the videos based on topic. An important consequence of this finding is that tagging games can provide a cost-effective alternative in situations when manual annotation by professionals is too costly.