In my opinion, the largest turn-offs for 1-time players are the graphics, in terms of maps. The models and other objects are alright, the marker models could be more interesting/attractive, but overall im not fussed.
I just know all the people I've introduced to it (apart from Reed, Sanch and WonTon) didn't like it due to the poor quality visuals. I think this is the problem of the map makers, which is why Reed and I try and step it up in that department. The engine - while limited - is capable of making good looking environments, especially with the reflective water (even though its a nightmare to use in conjunction with a detailed map). New initiatives by y00tz and others to push the envelope on map design and visuals should help to bridge the gap between other popular good looking Q2 mods, so while it's the number 1 problem for me, I don't think it deserves as much attention from the development team as the sounds, menu's/interface, etc...
The graphical style of DP is semi-serious, so the sounds are incongruous with that - a lot of new players often want realism and expect it from seeing screenshots, but then are presented with players whizzing round airtime with a barrage of simpsons soundclips, etc... TF2 might have a simple style by modern graphical standards, but it's art direction is absolutely fantastic, DP's is more confused I think.
I'm not saying I want a more serious game or anything, I like it the way it is, but the different aspects of the games presentation is out of sync and I feel it lowers the level of perceived quality for new players.
- Dagless