Can't say I've been to many terrible ones, but based on things I've found have and haven't worked well across a selection of them, I think the main things to keep in mind, whether you're going big or small and traditional or a bit alternative are:
-hold all aspects (ceremony, meal, evening party) in one place if possible. Failing that, in places that are close together and try to provide transport.
-avoid huge gaps between the different aspects. where there are gaps, try to ensure there's something to do and some form of food or drink.
-in particular, try to avoid a super-long photo session for the sake of both guests and the couple. You want a few lovely photos, but surely also want to have time to chat to people over a glass of champagne - and get some food before hours have passed
-conversely, if it's an event where everyone's staying in the same hotel or something, a gap between the meal and the evening party that's long enough for people to have a nap and/or get changed can be quite a good idea
-try to do the seating plan so guests are sat with people they know and like or, if they don't know many people, are likely to have at least something in common with
-allow partners if possible, particularly in the case of guests who don't know many other guests
-try to avoid inviting people neither of you really like or that you know are at risk of causing a scene
-make sure there's plenty of food throughout
-either provide plenty to drink or at least ensure there's a plentiful supply of reasonably priced drinks for people to purchase.
-provide as much detail as possible both with the invitations and on the day about timings, breaks, exactly who's invited, what is and isn't included etc
-try to ensure there's plenty of seating during the reception, for the benefit of both older guests and those who've been in stilettos all day!
-try to have indoor and outdoor space - or if outdoor focused, at least some shelter from both rain and boiling sun - and to keep indoor temperatures under control.
-try to keep the ceremony and the speeches to a reasonable length
- if you're having a big wedding but money is tight, aim to spend a bit more on the things that will make it actually fun for guests and a bit less on making yourself and the venue look pretty or on things that no one really cares about
- don't start too early in the morning, particularly if some guests have a long way to travel. but even if they don't, it's an exhausting day for people if you go right through from a mid-morning ceremony to a late night party
-the worst thing is when the bride and/or groom is clearly stressed or not enjoying themselves, so do whatever you need to do to avoid that, even if it contradicts some of the points above!!