Today's topic is gameplay. Now as you may know, gameplay is THE crucial part of a game that will either make it good, or bad. Surely the visual aspect of a game counts, as well as AI (if applicable), but gameplay is the most important part of a game, and that's why games should be focused on gameplay. Gameplay is not just one aspect, dozens of little things have to be taken into account for gameplay to be good. Let's go over a few.

First, there's the intuitive controls. Playing a video game, be it on PC or on a console, you like to be in control of things. The feeling that every action you take using your keyboard plus mouse, or controller, accurately reflects the actions performed in the game, is a first essential part of gameplay.
Secondly, there's the difficulty of a game. If a game is too simple, or too difficult, it's gameplay can be considered bad. The hard part with adjusting a game to a certain difficulty is that not every person who plays the game has the same amount of gaming experience/skill, as difficulty is a personal impression. Luckily, many games have the built-in option to adjust the game's difficulty to your own skill.
Thirdly, the entire visual/dynamic setup of the game must be believable. For instance, if you have a First Person Shooter game (FPS in short) in which you battle humans on loose platforms in the middle of space, it wouldn't be very believable. A naturesque setting in the woods, with a nice splashing waterfall and a rippling pond, however, make a much more enjoyable and plausible setting for such a game, given that the background story goes like that.
Those are all example aspects, if I covered all of them I'd still be typing here tomorrow, but the key with almost every single one of these elements is refinement. If an environment you can walk around in isn't refined, you'll find sloppy lack of detail or unfinished things you're not supposed to see, which we conveniently call bugs. If controls aren't defined, you can find the controls feel like you're walking on slippery ice, if the difficulty isn't refined enough, the game will only attract certain groups, either mainstream gamers, or their opposite, the hardcore gamers.
Now I'm going to be a little bit more specific, I'm going to discuss gameplay for one certain game, especially to please the UCMP team members who read this. Unreal Tournament 2004 is the game I'm currently designing maps for, and I'm lucky to be teamed up with a great bunch of people who have good knowledge of what they're doing, both with designing maps as in real life. All the sucking up aside, the UCMP team is focused on designing maps that primarily offer the gameplay players of the game are looking for in a map.
The gameplay dynamics for Unreal Tournament 2004 are very similar to most games within it's genre, which is FPS. Dynamic movement, balanced set of weapons/pickups, and action all around you. The key elements in bringing all this forward in a single map are easy to be listed, but often hard to live up to. To mention some of them; plenty of space, but not too much, plenty Z-axis, but again not too much, map layout with good flow, balanced and refined item/weapon placement, good AI, and a pretty sight to look at while playing. Though the last one I mentioned has hardly anything to do with gameplay, the believability of an environment often contributes to enjoying a match in that map. Taking all these things into account, you better know what you're doing when designing a map, because the community that receives it in the end can be very warm and friendly, but cold and hard as well.

Right, gameplay. I went through some of the main aspects of gameplay in the previous piece of text, now for application.

The first thing that you need to take into consideration, especially with a modern day FPS game like Unreal Tournament 2004, is that your layout shouldn't be less important than the way it looks. Therefore, creating a layout should primarily be done withOUT the visual appeal. This kind of strategy to build maps is used by hobbyist and professional companies alike, and can be called very efficient. The visual side of things is ALWAYS a less important aspect of a map than gameplay, and you'd do good to keep that in mind when you start mapping.
Getting a good layout means that you pay careful attention to several things. The first thing you should keep in mind is the scale. Unless you're purposefully making a 'mini-me' map (a map in which the player seems really small), you would be better off using a scale model for your map. With scale model I mean an object that represents the physical dimensions of a player. The next thing to do is to keep in account the movement for a certain game. In Unreal Tournament 2004, a player can double jump, dodge jump, wall dodge etc. These movements require some space to execute, but using these moves for useful shortcuts or trick jumps stimulates gameplay. These 2 things can really alter the way you look at map dimensions, for instance the distance between ledges can be done using a double jump plus walldodge from the left wall, or something like that. Rookies could play the map, but the more skilled players could play it too. Layout is probably THE most important aspect of gameplay. A good suggestion would be to have all sorts of players (skilled ones, trick jumpers, hardcore, mainstream, rookies) test your layout before you finalize it and move on to the next part. This goes for ALL gameplay related aspects by the way.