WOTC seems to be trying to attract old school players as well as 3.x players to D&D 5e. I think it would take most of the following — at a minimum — to attract large numbers of old school players and GMs to 5e.

1) Little to no 4e in the 5e core rules. The more they put 4e-like stuff in the core to satisfy the 4e fans, the less likely I suspect pre-4e players/GMs are to switch. This includes “abstract” things like too much stress on “by the numbers balance”, the weird 4e “we need our own IP” monsters, or designing the game around the needs of organized play (with the needs of “home play” seemingly an afterthought).

2) The 5e core rules need to be usable with all 0e-2e era settings and adventures with little to no advance conversion by the GM needed. With TSR D&D most GMs could use any adventure/setting with any TSR edition of D&D, performing whatever conversions were needed in his head. As a lot of old school players still use these settings and adventures, I believe this is pretty much “a must” to have any chance of wide adoption from old school players. However, old school players and GMs generally do not object to doing basic math in their heads (like converting between ascending and descending AC) which might make this easier to achieve. To get 3.x folks, 5e will need to be able to do this with 3.x materials as well. Although, as 3.x requires more GM prep to begin with, 5e can probably get by with some minor in-advance-of-play conversion from 3.x work needed. If 3e folks can’t use Paizo adventure paths and the like easily, I doubt many are likely to convert to 5e.

3) There must be an easy upgrade path to 5e. Old School groups will want to probably want to be able to easily convert their current campaigns and characters. As their current rules work, they are unlikely to be willing to end their current campaigns and start over just to use a new edition of D&D. Personally, I suspect 4e lost a number of players by simply being too different from what had gone before for a nearly seamless conversion of 3.x campaigns to be possible. 5e must not repeat this mistake.

4) The GM must “own the game rules”. The game must be complete in published rulebooks (purchasable — without a subscription — PDFs count as game books). The game must not be constantly changing via online errata/updates which are added without GM permission to an online character generators or other tools. Most old school players and GMs I know do not want their characters, settings, and/or adventures in constant flux because of reams of errata (or worse, retroactive rules changes needed to support new splatbooks or the like) are being added to the rules/tools they need to access at the whim of the publisher. Errata is great, but the choice to use it or not — on an item by item basis — needs to be up to the GM and the players in a given group.

5) The 5e system must be very friendly to both house rules and third party material. This would include any WOTC character generators and similar tools — players need to be able to easily add their house rules and third party rules. 5e must also have a third party license that, if not the OGL, is at least much closer to the OGL than to the 4e GSL. This license must be available to third party publishers no later than the day the first 5e book goes on sale.

However, to be blunt I think WOTC is going to have a very hard time getting a majority of old school players (and probably Pathfinder players as well) to drop their current system and use 5e. To do this, 5e will have to offer a much better game for their campaign, group, and style of play before they will actually switch — just “as good as” or “just a little better than” the rules they currently use will probably not get a large number of people to convert. Unfortunately, I have real doubts that WOTC can pull this off. 5e sounds like it may be a pretty good game, but I doubt it will be considerably better than what old school or Pathfinder players are playing now. If it isn’t, there will not be much reason to spend the money and time to convert to 5e.

However, all is not lost for WOTC even if I’m correct. If 5e can manage at least the first two items above, most of the adventures and settings WOTC might publish for 5e would be usable by players of other editions, which would mean WOTC would at least get some sales from some of the people playing older editions even if they don’t play 5e. This is something they did not get much of with 4e.