I'd say the biggest turn off for new players is the maps. While I honestly feel that castle1 is a great map and a map like blitz is not, new players want to actually PLAY... not walk around lost for 90 seconds and then get one-shotted by someone they didn't see or be ridiculed by people for being newbies and letting the other team cap. I also like the 60 second revive, but again, new players are here to PLAY... not sit in observer mode. There's a reason the most populated servers are the OTB speed server and syncmaster's cable -- it's because those servers play straight forward maps with very little downtime.
Personally, I think a team deathmatch mode would fix a lot of the frustration that new players feel because they would be pretty much assured instant action and reduced amount of other players complaining at them because hopefully they'll be too busy playing as well. Spawns would have to be true team deathmatch though (i.e., random spots across the map), because no one would like spawning in the same 6 spots over and over and over and over... especially if someone's just sitting there spraying the spawns (ub_cliff on OTB speed, anyone?)
Maps in general are also VERY far from being newbie friendly. Do you really expect someone completely new to the game to enjoy playing carpathian, airtime#, or pball_shreds#? I'm not knocking any of those maps for match play or even for pubs (lord knows I prefer them over stuff like blitz), but for the apparently 1000 new players downloading this game every day do you really think they would enjoy trying to figure out the controls while walking around and then getting shot by someone flying through the air at what must seem to be an impossible speed and height before they have a chance to react? Then, to top it all off, they have to sit in observer mode for 60 seconds before they have an opportunity to even test out any more controls/guns/items -- assuming someone doesn't spawn kill them, thereby putting them out of the action for another 60 seconds.
One of the best mapping books I ever got for Duke Nukem 3D -- which was the greatest game(play) ever... maybe you've heard of it? -- was The Official Duke Nukem 3D Level Design Handbook. I still have it, and not just for the nostalgic value. Appendix A in the book is Richard Gray's (aka Levelord -- the guy who did the majority of the duke3d maps) 17 Design Points for making a memorable level. I won't type the whole thing out since it's 8 full pages, but here are his points:
1. The main theme
2. Frame rate
3. Continuity
4. Details and realism
5. Appropriate enemies and weapons
6. There is no design point 6
7. The stick and the carrot (as applied to dp, this would be trick jumps -- but he takes care to mention that any particular reward should not be so overpowered as to make the challenge a necessity for the map to succeed)
8. Pick your audience and build around it (it's fine to make a match map, but server admins shouldn't be running maps like nhb in pub rotations -- not that any are now, i'm just making an example)
9. Player interaction
10. A well-balanced level
11. Pushing the envelope of the game engine (whoever made the first paintgren shooting canon, I commend you. That's a darn nifty idea.)
12. Patterns
13. Connections (views of areas yet to come -- i.e., windows)
14. Puzzles (the button to release your teammates from jail in routez is also a pretty darn cool idea)
15. Think sneaky (i don't think he was talking about hidden orgy rooms though...)
16. The fiddler's roof (N/A, it's a single player rule)
17. Practice what you preach (i'll quote this one in its entirety because apparently some people don't know it. "The last but definitely not the least good level-design point is play-testing. Play-test your level until you are sure you've checked everything. Play-test it some more, and then play-test it again. You should be completely sick of your level before you consider it finished. No matter what the target audience of your map will be -- single players, multiple players, or whoever -- play-test it under those exact conditions.")
The other official Duke 3d map designer, Allen Blum, had this helpful tip in his checklist, "Make the map . . . playable by everybody across all skill levels."
Okay, somehow this turned into a preaching post on map design, but I really do feel that the maps and general gameplay of DP are not tailored to new people at all. When you have a game that's one-shot-one-kill and there is such an incredibly high skill discrepancy between completely new players and people that have played for more than 3 months, it's going to be incredibly frustrating for a new player to run around on a map without any idea of where he is and then have a 60 second downtime once he finally encounters someone that either lands a single lucky shot on him or is just so much higher than his skill level that he didn't have a chance. I think a team deathmatch mode would be a tremendous help to alleviate some of this frustration that I'm sure new players feel.
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As for the graphics, I think you made a good decision in requiring an OpenGL card. Now I think you need to take it to the next step and include high-res textures with the game and have them turned on by default. You don't have to remove support for the old ones or not provide them anymore, but at least provide the GOOD ones with no seams in the distro and have high res textures turned on by default. I'm a big fan of both yours and FourthX's because you guys actually paid attention to whether seams in tiling is noticeable. Fullscreen should also be on by default and 800x600 resolution. I can't tell you how much more fun the game became when I dloaded a high-res texture pack and could appreciate how good the game actually looked for the first time in 10 years.
I also think the language barrier is an issue, but not much we can do about that.
I also think default.cfg should have "menu setup_controls" at the end of it so it's run the first time a new player loads the game if config.cfg doesn't exist yet.
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One last thing -- I stress maps and downtime so much because this is a free game. Players have nothing invested in it, and therefore are under no feeling of obligation to play it. Someone mentioned TF2 earlier, so I'll use that as an example -- if you went out and spent 50 bucks on the Orange Box to get TF2, are you going to quit it the first or second day because you didn't understand the gameplay or got frustrated at not being able to kill anything? Of course not -- you just spent 50 bucks. Now put yourself in the shoes of someone in that same frustrating situation when they downloaded a new game for free...