Redesigned Demo: It's just not up to date.

[Whirligig Girl]

First, let's take 0.25 as the starting point. (Although making a demo for any future version would work, though a few other things might need to be removed for it to be a good demo)

Destroy part of KSC

Remove Mission Control, Administration Building, and Spaceplane Hangars (But keep the ground, pool, and concrete there, so it feels like something is missing).

Gamemodes

Restrict the game to Science and Sandbox mode. "Career Mode is unavailable in this version". (Specifially "this version", not "the demo". It is usually less frustrating to not say "Demo", because it shoves in your face that a feature was removed. "This Version" is more subtle.)

Next, restrict the parts list.

Restricted Parts List

Command Pods

Mk1 Command Pod

Mk1 Lander Can

Probodobodyne OCTO probe core.

Propulsion

LV-909

LV-T45

LV-T30

RT-10 SRB

Rockomax BACC SRB

FL-T200

FL-T400

FL-T800

Oscar-B

LV-1

LV-1R

External Fuel Duct

RCS Fuel Tank

Spherical Small RCS Fuel Tank

Control

Reaction Wheel

RCS Thruster

Structural

Strut Connector

Short Truss

Long Truss

Truss Adapter

Octagonal Strut

Radial Decoupler

Radial Decoupler Large (The long one, not the flat one)

Stack Decoupler

Tiny Stack Decoupler

Aerodynamic

Nosecone

Static Winglet

Moving Winglet

Utility

Mk16 Parachute

Static Solar Panel

Small Battery

Stack battery bank

Landing Legs

Probe Landing Legs

Small Docking Port

Science

Thermometer

Mystery Goo

Communotron

Accelerometer

Science

Biomes are simplified. Kerbin has Ground, Ice, and Ocean, while Mun has Far-side and Near-side. Game balance so that the tech tee is completed by the time you've managed to land on the Mun and return to Kerbin.

Simplifed Tech Tree

Start

Mk1 Command Pod

FL-T400

LV-T30

Parachute

Communotron

General Rocketry

FL-T200

FL-T800

RT-10 SRB

Decoupler

Advanced Rocketry

LV-T45

Rockomax BACC SRB

LV-909

Landing

Landing Legs

Mk1 Lander Can

Small Truss

Flight Control

Winglet

Moving Winglet

Nosecone

Radial Decoupler

Large Radial Decoupler

Fuel Research

RCS Thruster

RCS Tank

Small Spherical RCS Tank

External Fuel Duct

Electronics

OCTO probe

Small Battery

Science

Mystery Goo

Thermometer

Static Solar Panel

Construction

Long Truss

Truss Adapter

Strut Connector

Small Docking Port

Octagonal Strut

Advanced Science

Accelerometer

Battery Bank

Reaction Wheel

Probes

Oscar-B

LV-1

LV-1R

Tiny Decoupler

Planes

Keep the runway intact. Allow the Aeris 3A only to be flown from that, and only one Aeris 3A can be flown at a time.

Spacecrafts

Only five crewed ships can be in flight at a time, but KerbalEVAs, debris and probe/satellites are fine.

Other

-Remove All planets aside from Kerbin, Mun, and Eeloo (But make it really hard to see, a sort of easter egg. It would be a real challenge to use just Demo parts to get to Eeloo!)

-Remove Asteroids

-Remove Custom Flags

-Remove Exploding Buildings

-Add an ending splash screen detailing the features of the full game, but pulled from the internet so it stays in touch with any major new features in updates after that. If disconnected from the internet, a default splash screen is shown.

-Allow all free versions of KSP to be downloaded, from 0.7.3 to 0.13.3, 0.18.3, and [0.25?].3 (Isn't it weird that the first public release, as well as all designated "Demo" versions have been 0.xx.3?]

Edited October 3, 2014 by GregroxMun

[Whirligig Girl]

They should just lie about the release date, then release it waaay earlier than people thought. EDIT: posted in wrong thread.

[Javster]

Not a bad idea, I personally think you can get any update for free provided there have been 5 major updates since. When 0.25 is released people can get 0.20, for example. With the really long times between updates, it's feasible.

No mods, because unless mod makers backdate by a lot, it is impossible.

[Red Iron Crown]

The demo definitely needs an update, it's far enough behind that it's not a great representation of the game any more. I would add some wing parts and at least one of the air-breathing engines to the parts list so that players can try their hand at making planes before purchasing.

[Whirligig Girl]

I don't think they should be able to make planes, just try flying one.

[Red Iron Crown]

Why not? Half the fun is building for some players. Give them a taste of plane building with a limited part set, just as is proposed with building rockets.

[Whirligig Girl]

I think it's just enough to keep players unsatisfied with the demo, but also make them want the full version for the airplanes.

[OrbitusII]

I do agree that the Demo could use an update, it's all the way back in 0.18. A demo update would be good if only to keep it relevant to the game's current state.

[Sirrobert]

I think it's just enough to keep players unsatisfied with the demo, but also make them want the full version for the airplanes.

The whole point of a Demo is to make people want to play the full game.

You do that by giving them a taste of the full game. A taste here is making a rocket/plane with limited parts, not an Airplane simualtion with an extremely weak aerodynamics model